Issue Description:

Here is simple example and command line to reproduce the issue:

// buggy.swift

import TensorFlow

public func foo() {
let maxCount: Int32 = 100
var a = Tensor<Int32>(0)
let b = Tensor<Int32>(1)
var count: Int32 = 0

a -= b
while count < maxCount {
a += b
count += 1
if count == 50 {
break
}
// a += b
}
a -= b
print(a)
}

foo()

build/Ninja-RelWithDebInfoAssert/swift-linux-x86_64/bin/swift -Xllvm --tf-dump-intermediates -Xllvm --tf-dump-graph -O buggy.swift

The crash is at:
SESE FIXME: Loops with multiple exits not handled yet!
UNREACHABLE executed at .../swift/lib/SILOptimizer/Mandatory/TFCanonicalizeCFG.cpp:261!

The input to partition pass is:

• • • • INPUT FUNCTION $S9B731014403fooyyF ----------
        // foo()
        sil @$S9B731014403fooyyF : $@convention(thin) () -> () {
        bb0:
        %0 = integer_literal $Builtin.Int32, 100 // user: %22
        %1 = integer_literal $Builtin.Int32, 0 // user: %2
        %2 = builtin "__tfop_tfc.scalarToTensor,$in"(%1 : $Builtin.Int32) : $TensorHandle<Int32> // users: %9, %8, %6
        %3 = integer_literal $Builtin.Int32, 1 // users: %19, %31, %4
        %4 = builtin "__tfop_tfc.scalarToTensor,$in"(%3 : $Builtin.Int32) : $TensorHandle<Int32> // users: %18, %16, %15, %13, %70, %44, %41, %40, %38, %30, %28, %27, %25, %10, %8, %7, %5
        strong_retain %4 : $TensorHandle<Int32> // id: %5
        strong_retain %2 : $TensorHandle<Int32> // id: %6
        strong_retain %4 : $TensorHandle<Int32> // id: %7
        %8 = builtin "__tfop_Sub,$in,$in"(%2 : $TensorHandle<Int32>, %4 : $TensorHandle<Int32>) : $TensorHandle<Int32> // users: %17, %16, %14
        strong_release %2 : $TensorHandle<Int32> // id: %9
        strong_release %4 : $TensorHandle<Int32> // id: %10
        %11 = integer_literal $Builtin.Int1, -1 // user: %31
        %12 = integer_literal $Builtin.Int32, 50 // user: %33
        strong_retain %4 : $TensorHandle<Int32> // id: %13
        strong_retain %8 : $TensorHandle<Int32> // id: %14
        strong_retain %4 : $TensorHandle<Int32> // id: %15
        %16 = builtin "__tfop_Add,$in,$in"(%8 : $TensorHandle<Int32>, %4 : $TensorHandle<Int32>) : $TensorHandle<Int32> // user: %19
        strong_release %8 : $TensorHandle<Int32> // id: %17
        strong_release %4 : $TensorHandle<Int32> // id: %18
        br bb1(%16 : $TensorHandle<Int32>, %3 : $Builtin.Int32) // id: %19

// %20 // users: %24, %29, %28, %26
// %21 // users: %31, %22
bb1(%20 : $TensorHandle<Int32>, %21 : $Builtin.Int32): // Preds: bb4 bb0
%22 = builtin "cmp_slt_Int32"(%21 : $Builtin.Int32, %0 : $Builtin.Int32) : $Builtin.Int1 // user: %23
cond_br %22, bb3, bb2 // id: %23

bb2: // Preds: bb1
br bb6(%20 : $TensorHandle<Int32>) // id: %24

bb3: // Preds: bb1
strong_retain %4 : $TensorHandle<Int32> // id: %25
strong_retain %20 : $TensorHandle<Int32> // id: %26
strong_retain %4 : $TensorHandle<Int32> // id: %27
%28 = builtin "__tfop_Add,$in,$in"(%20 : $TensorHandle<Int32>, %4 : $TensorHandle<Int32>) : $TensorHandle<Int32> // users: %35, %36
strong_release %20 : $TensorHandle<Int32> // id: %29
strong_release %4 : $TensorHandle<Int32> // id: %30
%31 = builtin "sadd_with_overflow_Int32"(%21 : $Builtin.Int32, %3 : $Builtin.Int32, %11 : $Builtin.Int1) : $(Builtin.Int32, Builtin.Int1) // user: %32
%32 = tuple_extract %31 : $(Builtin.Int32, Builtin.Int1), 0 // users: %35, %33
%33 = builtin "cmp_eq_Int32"(%32 : $Builtin.Int32, %12 : $Builtin.Int32) : $Builtin.Int1 // user: %34
cond_br %33, bb5, bb4 // id: %34

bb4: // Preds: bb3
br bb1(%28 : $TensorHandle<Int32>, %32 : $Builtin.Int32) // id: %35

bb5: // Preds: bb3
br bb6(%28 : $TensorHandle<Int32>) // id: %36

// %37 // users: %39, %41, %43
bb6(%37 : $TensorHandle<Int32>): // Preds: bb5 bb2
strong_retain %4 : $TensorHandle<Int32> // id: %38
strong_retain %37 : $TensorHandle<Int32> // id: %39
strong_retain %4 : $TensorHandle<Int32> // id: %40
%41 = builtin "__tfop_Sub,$in,$in"(%37 : $TensorHandle<Int32>, %4 : $TensorHandle<Int32>) : $TensorHandle<Int32> // users: %71, %62, %42
%42 = struct $Tensor<Int32> (%41 : $TensorHandle<Int32>) // user: %63
strong_release %37 : $TensorHandle<Int32> // id: %43
strong_release %4 : $TensorHandle<Int32> // id: %44
%45 = integer_literal $Builtin.Word, 1 // user: %48
%46 = integer_literal $Builtin.Int64, 1 // user: %47
%47 = struct $Int (%46 : $Builtin.Int64) // user: %52
%48 = alloc_ref [tail_elems $Any * %45 : $Builtin.Word] $_ContiguousArrayStorage<Any> // user: %49
%49 = upcast %48 : $_ContiguousArrayStorage<Any> to $_ContiguousArrayStorageBase // users: %59, %60, %58, %56, %54
%50 = integer_literal $Builtin.Int64, 2 // user: %51
%51 = struct $UInt (%50 : $Builtin.Int64) // user: %52
%52 = struct $_SwiftArrayBodyStorage (%47 : $Int, %51 : $UInt) // user: %53
%53 = struct $_ArrayBody (%52 : $_SwiftArrayBodyStorage) // user: %55
%54 = ref_element_addr %49 : $_ContiguousArrayStorageBase, #_ContiguousArrayStorageBase.countAndCapacity // user: %55
store %53 to %54 : $*_ArrayBody // id: %55
%56 = struct $_ContiguousArrayBuffer<Any> (%49 : $_ContiguousArrayStorageBase) // user: %57
%57 = struct $Array<Any> (%56 : $_ContiguousArrayBuffer<Any>) // user: %69
%58 = ref_tail_addr %49 : $_ContiguousArrayStorageBase, $Any // user: %61
strong_retain %49 : $_ContiguousArrayStorageBase // id: %59
strong_release %49 : $_ContiguousArrayStorageBase // id: %60
%61 = init_existential_addr %58 : $*Any, $Tensor<Int32> // user: %63
strong_retain %41 : $TensorHandle<Int32> // id: %62
store %42 to %61 : $*Tensor<Int32> // id: %63
// function_ref default argument 1 of print(_:separator:terminator🙂
%64 = function_ref @$Ss5print_9separator10terminatoryypd_S2StFfA0_ : $@convention(thin) () -> @owned String // user: %65
%65 = apply %64() : $@convention(thin) () -> @owned String // user: %69
// function_ref default argument 2 of print(_:separator:terminator🙂
%66 = function_ref @$Ss5print_9separator10terminatoryypd_S2StFfA1_ : $@convention(thin) () -> @owned String // user: %67
%67 = apply %66() : $@convention(thin) () -> @owned String // user: %69
// function_ref print(_:separator:terminator🙂
%68 = function_ref @$Ss5print_9separator10terminatoryypd_S2StF : $@convention(thin) (@owned Array<Any>, @owned String, @owned String) -> () // user: %69
%69 = apply %68(%57, %65, %67) : $@convention(thin) (@owned Array<Any>, @owned String, @owned String) -> ()
strong_release %4 : $TensorHandle<Int32> // id: %70
strong_release %41 : $TensorHandle<Int32> // id: %71
%72 = tuple () // user: %73
return %72 : $() // id: %73
} // end sil function '$S9B731014403fooyyF'

• • • • END OF INPUT FUNCTION ----------