[llvm] [GlobalISel] Check whether `G_CTLZ` is legal in `matchUMulHToLShr` (PR #126457)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

Can you also add and end to end IR test where this reproduced 

https://github.com/llvm/llvm-project/pull/126457