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

[Emil J via llvm-commits]

ekliptik wrote:

Looks like ths PR and issue are unlinked. This PR fixes #77762. I will describe the problem in clearer terms in the issue

- [ ] Add test

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