[AliasAnalysis] Treat invariant.start as read-memory

Summary:
We teach alias analysis that invariant.start is readonly.
This helps with GVN and memcopy optimizations that currently treat.
invariant.start as a clobber.
We need to treat this as readonly, so that DSE does not incorrectly
remove stores prior to the invariant.start

Reviewers: sanjoy, reames, majnemer, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23214

llvm-svn: 278138