[CodeGen] Refactor MachineMemOperand::Flags's target-specific flags.

Summary:
Make the target-specific flags in MachineMemOperand::Flags real, bona
fide enum values.  This simplifies users, prevents various constants
from going out of sync, and avoids the false sense of security provided
by declaring static members in classes and then forgetting to define
them inside of cpp files.

Reviewers: MatzeB

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22372

llvm-svn: 275451