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

[Matt Arsenault via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
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arsenm wrote:

You would have an easier time reproducing this by directly emitting a multiply + shift by half bitwidth. it just happens one appears inside the div expansion 

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