[AArch64][GlobalISel] Move imm adjustment for G_ICMP to post-legalizer lowering

Move the code which adjusts the immediate/predicate on a G_ICMP to
AArch64PostLegalizerLowering.

This

- Reduces the number of places we need to test for optimized compares in the
selector. We know that the compare should have been simplified by the time it
hits the selector, so we can avoid testing this in selects, brconds, etc.

- Allows us to potentially fold more compares (previously, this optimization
was only done after calling `tryFoldCompare`, this may allow us to hit some more
TST cases)

- Simplifies the selection code in `emitIntegerCompare` significantly; we can
just use an emitSUBS function.

- Allows us to avoid checking that the predicate has been updated after
`emitIntegerCompare`.

Also add a utility header file for things that may be useful in the selector
and various combiners. No need for an implementation file at this point, since
it's just one constexpr function for now. I've run into a couple cases where
having one of these would be handy, so might as well add it here. There are
a couple functions in the selector that can probably be factored out into
here.

Differential Revision: https://reviews.llvm.org/D89823