[llvm] [GlobalISel] Add G_ABS computeKnownBits (PR #154413)

[Pragyansh Chaturvedi via llvm-commits]

r41k0u wrote:

I came across another issue while working on this, I will add a fix for that (and I must add a sign extension test case as well).

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