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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

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

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