[AArch64][GlobalISel] Fold 64-bit cmps with 64-bit adds
G_ICMP is selected to an arithmetic overflow op (ADDS/SUBS/etc) with a dead
destination + a CSINC instruction.
We have a fold which allows us to combine 32-bit adds with G_ICMP.
The problem with G_ICMP is that we model it as always having a 32-bit
destination even though it can be a 64-bit operation. So, we were missing some
opportunities for 64-bit folds.
This patch teaches the fold to recognize 64-bit G_ICMPs + refactors some of
the code surrounding CSINC accordingly.
(Later down the line, I think we should probably change the way we handle G_ICMP
in general.)
Differential Revision: https://reviews.llvm.org/D111088