[llvm] [ValueTracking] Let ComputeKnownSignBits handle (shl (zext X), C) (PR #97693)

[Björn Pettersson via llvm-commits]

bjope wrote:

Replaced unit test case with a lit based test (thanks to @goldsteinn for great tip about the and+add fold!).

https://github.com/llvm/llvm-project/pull/97693