[SafepointIRVerifier] Add verifier pass for finding GC relocation bugs

Original Patch and summary by Philip Reames.

RewriteStatepointsForGC tries to rewrite a function in a manner where
the optimizer can't end up using a pointer value after it might have
been relocated by a safepoint. This pass checks the invariant that
RSForGC is supposed to establish and that (if we constructed semantics
correctly) later passes must preserve.

This has been a really useful diagnostic tool when initially developing
the rewriting scheme and has found numerous bugs.

Differential Revision: https://reviews.llvm.org/D15940

Reviewed by: swaroop.sridhar, mjacob

Subscribers: llvm-commits
llvm-svn: 307112