[SafepointIRVerifier] Allow deriving pointers from unrelocated base

Summary:
This patch allows to use derived pointers (GEPs/bitcasts) of unrelocated
base pointers. We care only about the uses of these derived pointers.

It is acheived by two changes:
1. When we have enough information to say if the pointer is unrelocated at some
point or not, we walk all BBs to remove from their Contributions all valid defs
of unrelocated pointers (GEP with unrelocated base or bitcast of unrelocated
pointer).
2. When it comes to verification we just ignore instructions that were removed
at stage 1.

Patch by Daniil Suchkov!

Reviewers: anna, reames, apilipenko, mkazantsev

Reviewed By: anna, mkazantsev

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40289

llvm-svn: 319838