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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@AZero13 It'd be better if we can determine the general missing DAG fold instead of micro-optimizing with custom lowering like this.

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