Issue Description:

I get the following error when trying to compile this package: https://github.com/artman/Signals (I am not the author)

/swift/usr/bin/swift build -v (in directory: /Signals)
lsb_release -r
which clang
/swift/usr/bin/swiftc --driver-mode=swift -L /swift/usr/lib/swift/pm/3 -lPackageDescription -swift-version 3 -I /swift/usr/lib/swift/pm/3 -sdk / /Signals/Package.swift -fileno 6
/swift/usr/bin/swift-build-tool -f /Signals/.build/debug.yaml main -v
/swift/usr/bin/swiftc -module-name Signals -incremental -emit-dependencies -emit-module -emit-module-path /Signals/.build/x86_64-unknown-linux/debug/Signals.swiftmodule -output-file-map /Signals/.build/x86_64-unknown-linux/debug/Signals.build/output-file-map.json -parse-as-library -num-threads 8 -c /Signals/Sources/Signal.swift -I /Signals/.build/x86_64-unknown-linux/debug -swift-version 3 -target x86_64-unknown-linux -sdk / -Onone -g -enable-testing -j8 -DSWIFT_PACKAGE -module-cache-path /Signals/.build/x86_64-unknown-linux/debug/ModuleCache
swift: /home/buildnode/jenkins/workspace/oss-swift-4.0-package-linux-ubuntu-16_04/llvm/lib/IR/Instructions.cpp:2592: static llvm::CastInst *llvm::CastInst::Create(Instruction::CastOps, llvm::Value *, llvm::Type *, const llvm::Twine &, llvm::Instruction *): Assertion `castIsValid(op, S, Ty) && "Invalid cast!"' failed.
0 swift 0x0000000003ae9918
1 swift 0x0000000003aea056
2 libpthread.so.0 0x00007f602c079390
3 libc.so.6 0x00007f602a7b1428 gsignal + 56
4 libc.so.6 0x00007f602a7b302a abort + 362
5 libc.so.6 0x00007f602a7a9bd7
6 libc.so.6 0x00007f602a7a9c82
7 swift 0x00000000039a6711
8 swift 0x00000000004d5f9b
9 swift 0x0000000000507245
10 swift 0x00000000005046f1
11 swift 0x00000000005c8c8e
12 swift 0x00000000005b4855
13 swift 0x00000000005b223d
14 swift 0x00000000004cb29e
15 swift 0x000000000058d15c
16 swift 0x000000000058d957
17 swift 0x00000000004ae518
18 swift 0x00000000004aa3eb
19 swift 0x0000000000465424
20 libc.so.6 0x00007f602a79c830 __libc_start_main + 240
21 swift 0x0000000000462ce9
Stack dump:
0. Program arguments: /swift/usr/bin/swift -frontend -c -primary-file /Signals/Sources/Signal.swift -target x86_64-unknown-linux -disable-objc-interop -sdk / -I /Signals/.build/x86_64-unknown-linux/debug -enable-testing -g -module-cache-path /Signals/.build/x86_64-unknown-linux/debug/ModuleCache -swift-version 3 -D SWIFT_PACKAGE -emit-module-doc-path /Signals/.build/x86_64-unknown-linux/debug/Signals.build/Signalpartial.swiftdoc -Onone -parse-as-library -module-name Signals -emit-module-path /Signals/.build/x86_64-unknown-linux/debug/Signals.build/Signalpartial.swiftmodule -emit-dependencies-path /Signals/.build/x86_64-unknown-linux/debug/Signals.build/Signal.d -emit-reference-dependencies-path /Signals/.build/x86_64-unknown-linux/debug/Signals.build/Signal.swiftdeps -num-threads 8 -o /Signals/.build/x86_64-unknown-linux/debug/Signals.build/Signal.swift.o

1. While emitting IR SIL function "@_T07Signals6SignalC18cancelSubscriptionyyXl3for_tF".
   for 'cancelSubscription(for:)' at /Signals/Sources/Signal.swift:140:12
   :0: error: unable to execute command: Aborted
   :0: error: compile command failed due to signal 6 (use -v to see invocation)
   error: terminated(1): /swift/usr/bin/swift-build-tool -f /Signals/.build/debug.yaml main -v
   Compilation failed.

The function in question:

/// Cancels all subscriptions for an observer.
///
/// - parameter observer: The observer whose subscriptions to cancel
public func cancelSubscription(for observer: AnyObject) {
signalListeners = signalListeners.filter {
if let definiteListener:AnyObject = $0.observer {
return definiteListener !== observer
}
return false
}
}

If I comment out the identity comparison and just have the function return a Bool then it compiles fine.