swift234

Implement a first hack at (limited) alloc_stack to register 
promotion.  This pass is destined to be somewhat different than
LLVM's mem2reg pass:
 - Because we can't break debug information, this won't remove
   alloc_stack (and the stores to it) for allocations with location info.
 - Because this gates other dataflow diagnostics, we don't want a value
   being pinned to the stack to prevent promotion.  Instead, we promote
   all loads that we can, even if some end up being blocked by a [byref]
   "may" store.

Lots of caveats herein:
 - This doesn't do cross block promotion yet.
 - This doesn't remove the allocation even when it is allowed to.
 - This causes a few IRGen test failures, including an irgen crash.
 - This should be split out to its own file, its own pass, and its own
   testcase.

Because of these caveats, this is disabled by default right now. Pass -enable-stack-promotion to get it.


Swift SVN r6454