Issue Description:

Expected behavior: a function that directly uses an inout parameter is exactly equivalent to one that copies the inout variable into a local at the beginning then copies it back out at the end.

Actual behavior: the code that uses the inout parameter directly is an order of magnitude slower than the code that manually copies the inout variable.

Tested in Apple Swift 5.0

// Doesn't optimize.
func fx(_ allints: inout [Int],  _ j : inout Int) {
      for k in allints.indices {
        allints[k] = j
        j &+= 1
      }
 }

// Does optimize.
 func f(_ allints: inout [Int],  _ i : inout Int) {
      var j = i
      for k in allints.indices {
        allints[k] = j
        j &+= 1
      }
      i = j
 }