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

[Amara Emerson via llvm-commits]

aemerson wrote:

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

It's fine actually, I'm going to refactor some of this to use the new `GPhi` wrapper recently added.

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