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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> I agree this should be done in DAGCombiner.

@arsenm Any comments?


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