[DAG] computeKnownBits - abs(x) will be zero in the upper bits if x is sign-extended (#94382)

As reported on https://github.com/llvm/llvm-project/issues/94344 - if x has more than one signbit, then the upper bits of its absolute value are guaranteed to be zero

Alive2: https://alive2.llvm.org/ce/z/a87fHU

Fixes #94344