[llvm] [InstCombine] Fold extended add/sub of the same type (PR #185259)

[Andreas Jonson via llvm-commits]

https://github.com/andjo403 commented:

see https://llvm.org/docs/InstCombineContributorGuide.html for some guidens about tests to add and about adding proof for the folds in the description

https://github.com/llvm/llvm-project/pull/185259