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

[Nikita Popov via llvm-commits]

nikic wrote:

CGP should only be used for transforms that require cross-block reasoning, which does not seem to be the case here. Aspirationally GlobalISel does not need CGP at all, because it can perform those optimizations itself. (Realistically, we are far from that...)

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