swift234

COWArrayOpt: Handle cases where the array is not uniquely identified

We can still hoist in many cases if all operations in the loop are 'safe' wrt.
hoisting.

'Safe' array operations are

  * all array semantic functions (as long as one array type cannot contain
    another).
  * stores to array elements
  * any instruction that does not have side effects.
  * any retain must be matched by a release before we hit a make_unique.

rdar://19668784

Swift SVN r24960