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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Please can you look at the failing AArch64 test:
```
Failed Tests (1):
  LLVM :: CodeGen/AArch64/abd-combine.ll
```

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