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

[David Green via llvm-commits]

https://github.com/davemgreen commented:

I think considering these are fairly AArch64 specific instructions, it might be best to just make this custom, in the same way we have it for SDAG.

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