[llvm] [ValueTracking] Use select condition to help infer bits of arms (PR #84699)

[via llvm-commits]

goldsteinn wrote:

> > The implementation looks correct to me now, but unfortunately this has also made it too expensive: http://llvm-compile-time-tracker.com/compare.php?from=1bee754fc62ed053e347891e8967b645e95ddbae&to=a4b5288b2bdd57cc31551977d5413abbb95413a9&stat=instructions:u
> 
> cest la vie
> 
> Ill see if I can speed it up

Moved the undef check to the very end, seems to handle the bulk of the compile time regressions:
https://llvm-compile-time-tracker.com/compare.php?from=fa1d13590cfd0be77c452cca929bc32efb456627&to=bc2e2bbec067214c464a43cf632d287469143e29&stat=instructions%3Au

https://github.com/llvm/llvm-project/pull/84699