swift234

Fix placement of alloc_stack/dealloc_stack

Update the box-to-stack promotion pass to place the newly promoted
allocations at the function entry, and deallocations before each return.

Ideally we would scope these lifetimes down, but it's a bit challenging
because:
  - The capture promotion pass can insert releases, and these releases
    are not in any particular order with respect to the box creation.
  - The box-to-stack promotion pass has for a while now been promoting
    boxes that are arguments to partial_apply when we know the
    partial_apply doesn't escape the current function. To correctly
    place the deallocs, we would need to determine the potential
    lifetimes of the allocated data, e.g. the last use of the
    partial_apply along any given path.

I have opened <rdar://problem/16723128> so that we can try to improve
this in the future.

Fixes <rdar://problem/16242937>.

Swift SVN r16852