[llvm] [SDAG] Combine select into ABD?, for const (PR #173581)

[via llvm-commits]

DaKnig wrote:

@RKSimon please review again, now the code uses the (now merged) m_SpecificNeg.

In the future how can I activate CI without bothering other devs? I plan to upload more patches and sometimes it would be faster for me to upload to CI and let it test things than to rebase/compile locally/push 

https://github.com/llvm/llvm-project/pull/173581