Folding compares with unescaped allocations

Summary:
If we know that the pointer allocated within a function does not escape,
we can fold away comparisons that are done with global pointers

Patch by Anna Thomas!

Reviewers: reames, majnemer, sanjoy

Subscribers: mgrang, mcrosier, majnemer, llvm-commits

Differential Revision: http://reviews.llvm.org/D19276

llvm-svn: 267035