[SafepointIRVerifier] Avoid false positives in GC verifier for compare between pointers

Today the safepoint IR verifier catches some unrelocated uses of base
pointers that are actually valid.

With this change, we narrow down the set of false positives.
Specifically, the verifier knows about compares to null and compares
between 2 unrelocated pointers.

Reviewed by: skatkov

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35057

llvm-svn: 307392