[PowerPC] Fix a number of inefficiencies and issues with atomic code gen

There are a few issues with the code we generate for atomic operations and the way we generate it:

- Hard coded CR0 for compares
- Order of operands for compares not conducive to
  emitting compare-immediate or for CSE of compares
- Missing MachineMemOperand for st[bhwd]cx intrinsics
- Missing intrinsic properties for the same
- Unnecessary blocks with store conditional
  instructions to clear reservation (which ends
  up hindering performance)
- Move from CR instructions just to compare the
  result of a store conditional with zero (even
  though it is a record-form)

This patch aims to resolve all of those issues.

Differential revision: https://reviews.llvm.org/D134783