Strip off invariant.start because memory locations arent invariant

The original change was reverted in rL317217 because of the failure in
the RS4GC testcase. I couldn't reproduce the failure on my local machine
(macbook) but could reproduce it on a linux box.

The failure was around removing the uses of invariant.start. The fix
here is to just RAUW undef (which was the first implementation in D39388).
This is perfectly valid IR as discussed in the review.

llvm-svn: 317225