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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> slightly surprised this didn't fire in any amdgpu tests

The legality checks is limiting this in a number of places - I did look at relaxing it but it started to cause hangs.

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