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

[Pragyansh Chaturvedi via llvm-commits]

r41k0u wrote:

The cache assertion in `getKnownBits` looks like a temporary thing until the function is fleshed out (although you'd know more about this). And since it is being added to computeNumSignBits as well, I wonder if that function can benefit from the cache somehow as well.

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