[AArch64][GlobalISel] Teach tryOptSelect to handle G_ICMP

This teaches `tryOptSelect` to handle folding G_ICMP, and removes the
requirement that the G_SELECT we're dealing with is floating point.

Some refactoring to make this work nicely as well:

- Factor out the scalar case from the selection code for G_ICMP into
  `emitIntegerCompare`.
- Make `tryOptCMN` return a MachineInstr* instead of a bool.
- Make `tryOptCMN` not modify the instruction being selected.
- Factor out the CMN emission into `emitCMN` for readability.

By doing this this way, we can get all of the compare selection optimizations
in select emission.

Differential Revision: https://reviews.llvm.org/D64084

llvm-svn: 364961