[llvm] [GISel] Fix #77762: extend correct source registers in combiner helper rule extend_through_phis (PR #77765)

[Amara Emerson via llvm-commits]

aemerson wrote:

Thanks for the fix. This needs a test, and it's not immediately clear to me why this change is correct vs the old code?

https://github.com/llvm/llvm-project/pull/77765