[NFC] Refactor SafepointIRVerifier

Now two classes are responsible for verification: one of them can track GC
pointers and know whether a pointer is relocated or not and another based on
that information can verify uses of GC pointers.

Patch Author: Daniil Suchkov
Reviewers: mkazantsev, anna, apilipenko
Reviewed By: mkazantsev
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D40885

llvm-svn: 320549