[llvm] [ARM][AArch64] Custom Lower abds and abd on ARM and AArch64 (PR #155884)

[Eli Friedman via llvm-commits]

https://github.com/efriedma-quic commented:

CC @RKSimon 

It looks like the improvements are mostly not from the lowering itself.  The lowering is, as far as I can tell, basically what TargetLowering::expandABD would generate anyway, at least on AArch64.  The difference is that certain DAGCombines trigger more aggressively.

Given that, we probably should change the conditions for those DAGCombines, instead of adding a bunch of useless custom lowering code.

https://github.com/llvm/llvm-project/pull/155884