[Verifier] Assert gc_relocate always return a pointer type

Summary: Add an assertion in verifier.cpp to make sure gc_relocate relocate a gc pointer, and its return type has the same address space with the relocated pointer.

Reviewers: reames, AndyAyers, sanjoy, pgavlin

Reviewed By: pgavlin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9695

llvm-svn: 237605