[llvm] [GlobalISel] Add G_ABS computeKnownBits, add ComputeKnownBitsCache assertion to computeNumSignBits (PR #154413)

[David Green via llvm-commits]

https://github.com/davemgreen commented:

Thanks - It looks like there is another one that goes computeNumSignBitsImpl -> getValidMinimumShiftAmount -> getValidShiftAmountRange -> getKnownBits.

I'm wondering if it makes sense to keep the cache, or whether it is doing more harm than good. I tried checking the compile time and it wasn't very useful (although the hit rate was relatively good). I will try and put up a patch that removes it and see what people think.

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