Fold compares irrespective of whether allocation can be elided

Summary
When a non-escaping pointer is compared to a global value, the
comparison can be folded even if the corresponding malloc/allocation
call cannot be elided.
We need to make sure the global value is not null, since comparisons to
null cannot be folded.

In future, we should also handle cases when the the comparison
instruction dominates the pointer escape.

Reviewers: sanjoy
Subscribers s.egerton, llvm-commits

Differential Revision: http://reviews.llvm.org/D19549

llvm-svn: 268390