[PATCH] Implement variable-sized alloca instrumentation.

[Kostya Serebryany]

Interesting. 
The dynamic alloca in this case is moved to a basic block outside of main path and hence it does not dominate 
all exits. So, we can not unpoison it at the RET statements, instead we should do it in the end of the alloca's scope.
This is starting to resemble use-after-scope... Alexey, any comments?

http://reviews.llvm.org/D6055