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

[Simon Pilgrim via llvm-commits]

RKSimon 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. Stacking this PR would have been better to see the changes properly?

Happy for you to stack them - cheers

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