[llvm] [DAGCombine] Count leading ones: refine post DAG/Type Legalisation if promotion (PR #102877)

[via llvm-commits]

v01dXYZ wrote:

> could you split this PR into at least two separate ones

I'll do that as the SDPatternMatch seems to require work on its own.

https://github.com/llvm/llvm-project/pull/102877