[GVN] Don't coerce non-integral pointers to integers or vice versa

Summary:
See http://llvm.org/docs/LangRef.html#non-integral-pointer-type

The NewGVN test does not fail without these changes (perhaps it does
try to coerce pointers <-> integers to begin with?), but I added the
test case anyway.

Reviewers: dberlin

Subscribers: mcrosier, llvm-commits, Prazek

Differential Revision: https://reviews.llvm.org/D32208

llvm-svn: 300730