[EarlyCSE] Optionally use MemorySSA. NFC.

[EarlyCSE] Optionally use MemorySSA. NFC.

Summary:
Use MemorySSA, if requested, to do less conservative memory dependency
checking.

This change doesn't enable the MemorySSA enhanced EarlyCSE in the
default pipelines, so should be NFC.

Reviewers: dberlin, sanjoy, reames, majnemer

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19821

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280279 91177308-0d34-0410-b5e6-96231b3b80d8