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

[Björn Pettersson via llvm-commits]

bjope wrote:

Added some test cases based on the `(sshlsat x, c) -> (shl x, c)` fold to verify number of sign bits that are computed.

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