[llvm] [DAG] computeKnownBits - add ISD::ABDU/ISD::ABDS handling #84905 (PR #88253)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

Please can you add the new tests as the first commit in the patch, and the handling (and test changes) as the second commit so it can show the effect

https://github.com/llvm/llvm-project/pull/88253