[RS4GC] Strip off invariant.start because memory locations arent invariant

Summary:
Invariant.start on memory locations has the property that the memory
location is unchanging. However, this is not true in the face of
rewriting statepoints for GC.
Teach RS4GC about removing invariant.start so that optimizations after
RS4GC does not incorrect sink a load from the memory location past a
statepoint.

Added test showcasing the issue.

Reviewers: reames, apilipenko, dneilson

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39388

llvm-svn: 317215