[llvm] [DAG] Fold (sext (sext_inreg x)) -> (sext (trunc x)) if the trunc is free (PR #77616)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

slightly surprised this didn't fire in any amdgpu tests 

https://github.com/llvm/llvm-project/pull/77616