[llvm] [DAGCombiner] fold `(sext_inreg (freeze (extload x)))` -> `(freeze (sextload x))` (PR #226208)

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VachanVY wrote:

@RKSimon I've verified that this fold fixes the regression as you can see in the above debug files, but I'm stuck with writing tests for this fold.

https://github.com/llvm/llvm-project/pull/226208