[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:

> Does this need to happen in CGP for some reason, or would DAGCombine also work?

I implement this in CGP to avoid duplicating the logic in GISel.
See also the comment https://github.com/llvm/llvm-project/pull/81404#discussion_r1487117050.


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