[llvm] [GlobalISel] Add G_ABS computeKnownBits, add computeNumSignBitsImpl to work ComputeKnownBitsCache assertions (PR #154413)

[David Green via llvm-commits]

https://github.com/davemgreen approved this pull request.

Thank you, LGTM.

The title could do with an update now, otherwise are you happy for this to be submitted?

https://github.com/llvm/llvm-project/pull/154413