[TargetLowering] Add hook for adding target MMO flags when doing ISel.

Summary: Add TargetLowering hook getMMOFlags() to add target specific
MMO flags to load/store instructions created by ISel.

Reviewers: bogner, hfinkel, qcolombet, MatzeB

Subscribers: mcrosier, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D34962

llvm-svn: 307879