[llvm] Optimize count leading ones if promoted type (PR #99591)

[via llvm-commits]

v01dXYZ wrote:

@topperc Currently, the type legaliser not only extends the operands but needs to add a substract node too. Using a DAGCombiner would mean detect this pattern and reverse it instead of trying to prevent to create it (as it is in this PR). I understand your point though as this PR adds another exception to Type Legalisation. I'm not experienced enough to have a worthy opinion about which one is better.

I'll try to implement it with a DAGCombiner in another PR.

https://github.com/llvm/llvm-project/pull/99591