IR: Fold away compares between GV GEPs and GVs

A GEP of a non-weak global variable will not be equivalent to another
non-weak global variable or a GEP of such a variable.

Differential Revision: http://reviews.llvm.org/D4238

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212360 91177308-0d34-0410-b5e6-96231b3b80d8