[AArch64][GlobalISel] Use TST for comparisons when possible

Porting over the part of `emitComparison` in AArch64ISelLowering where we use
TST to represent a compare.

- Rename `tryOptCMN` to `tryFoldIntegerCompare`, since it now also emits TSTs
  when possible.

- Add a utility function for emitting a TST with register operands.

- Rename opt-fold-cmn.mir to opt-fold-compare.mir, since it now also tests the
  TST fold as well.

Differential Revision: https://reviews.llvm.org/D64371

llvm-svn: 365404