[llvm] [AArch64][GlobalISel] Take abs scalar codegen closer to SDAG (PR #84886)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm @aemerson Given that there is a different approach now, do you still think `lower()` should be modified? If so, how do we reliably detect? Can you please elaborate a bit more?

Yes. Currently the lower implementation just calls lowerAbsToAddXor. You can conditionally call your new implementation based on isLegalOrCustom for the type with the relevant opcodes used in the expansion 

https://github.com/llvm/llvm-project/pull/84886