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

[Amara Emerson via llvm-commits]

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

LGTM, can you fix the clang format issue before you merge?

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