[MemorySSA] Add new tests for invariant.groups

Summary:
Next round of extra tests for MSSA.
I have a prototype invariant.group handling implementation
that fixes all the FIXMEs, and I think it will be
easier to see what is the difference if I firstly
post this, and then only fix fixits.

Reviewers: george.burgess.iv, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29022

llvm-svn: 292797