[AArch64][GlobalISel] Split post-legalizer combiner to allow for lowering at -O0

[AArch64][GlobalISel] Split post-legalizer combiner to allow for lowering at -O0

There are a lot of combines in AArch64PostLegalizerCombiner which exist to
facilitate instruction matching in the selector. (E.g. matching for G_ZIP and
other shuffle vector pseudos)

It still makes sense to select these instructions at -O0.

Matching earlier in a combiner can reduce complexity in the selector
significantly. For example, a good portion of our selection code for compares
would be a lot easier to represent in a combine.

This patch moves matching combines into a "AArch64PostLegalizerLowering"
combiner which runs at all optimization levels.

Also, while we're here, improve the documentation for the
AArch64PostLegalizerCombiner, and fix up the filepath in its file comment.

And also add a 'r' which somehow got dropped from a bunch of function names.

https://reviews.llvm.org/D89820