swift234

Handle multiple releases in allocbox-to-stack.

Promote alloc_box to alloc_stack in the case where we have multiple
releases along different paths (including paths that follow a back-edge
to the alloc_box itself).

The new approach effectively uses liveness (computed on a per-alloc_box
basis) to determine where the final release on any path will
be. Building the stdlib takes the same amount of time as it did before
this change.

Deletes 56 of the 116 remaining alloc_box in the stdlib, and speeds up
the string walk benchmark by ~8%.

Swift SVN r15322