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

[Madhur Amilkanthwar via llvm-commits]

madhur13490 wrote:

> 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.

Do you mean just use `customifScalar` and not have anything in `lower()`?

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