[CodeGen] Refactor MachineMemOperand's Flags enum.

Summary:
- Give it a shorter name (because we're going to refer to it often from
  SelectionDAG and friends).

- Split the flags and alignment into separate variables.

- Specialize FlagsEnumTraits for it, so we can do bitwise ops on it
  without losing type information.

- Make some enum values constants in MachineMemOperand instead.
  MOMaxBits should not be a valid Flag.

- Simplify some of the bitwise ops for dealing with Flags.

Reviewers: chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D22281

llvm-svn: 275438