[EarlyCSE] Teach about CSE'ing over invariant.start intrinsics

Summary:
Teach EarlyCSE about invariant.start intrinsic. Specifically, we can perform
store-load, load-load forwarding over this call.

Reviewers: majnemer, reames, dberlin, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23268

llvm-svn: 278153