[llvm] [DAGCombine] Transform `shl X, cttz(Y)` to `mul (Y & -Y), X` if cttz is unsupported (PR #85066)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Please move this into DAGCombine (and GISel if you want to handle both)

Done (only for DAGCombine).

https://github.com/llvm/llvm-project/pull/85066