swift234

Remove alloc_ref with simple destructors and only store users.

This pass attempts to remove alloc_ref and everything that uses the alloc_ref
if:

1. The alloc_ref has a destructor which we can very does not have escaping side
effects.
2. The alloc_ref does not have any non-trivial uses that are not stores.

It reduces ObjInst on my cpu from 10206922128 ns to 46 ns (i.e. nothing).

Swift SVN r12990