[llvm] [SelectionDAG] Let ComputeKnownSignBits handle (shl (ext X), C) (PR #97695)

[Björn Pettersson via llvm-commits]

bjope wrote:

No regression tests yet. If anyone has a good idea how and where to put them, let me know.

The need for this was found downstream as DAG Combiner prefer `(SHL (ANY_EXTEND (ASHR ...)))` instead of `(SHL (SIGN_EXTEND (ASHR ...)))`, which actually is bad for ComputeNumSignBits.

https://github.com/llvm/llvm-project/pull/97695