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

[Wang Pengcheng via llvm-commits]

https://github.com/wangpc-pp commented:

I agree this should be done in DAGCombiner.

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