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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > I agree this should be done in DAGCombiner.
> 
> @arsenm Any comments?

Yes, this is straightforward combining. The downside is then you have to do it twice, in the DAG and GISel 

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