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

[David Green via llvm-commits]

davemgreen wrote:

Given we have a csneg instruction, do we just want `cmp x, 0; csneg x, x`? i.e I would expect us to want to lower to a compare+neg+select (which isn't either of the existing lowerAbs* methods, and might not be easy to tell generically that we want to generate code for, if there is not an obvious way to check for csneg).

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