Add MemorySSA as loop dependency, disabled by default [NFC].

Summary:
First step in adding MemorySSA as dependency for loop pass manager.
Adding the dependency under a flag.

New pass manager: MSSA pointer in LoopStandardAnalysisResults can be null.
Legacy and new pass manager: Use cl::opt EnableMSSALoopDependency. Disabled by default.

Reviewers: sanjoy, davide, gberry

Subscribers: mehdi_amini, Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D40274

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@318772 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed