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

[Emil J via llvm-commits]

ekliptik wrote:

Oops, I was manually renaming the variables since I wrote and verified the test when rebased on our fork which had an older version of the check statements. I pushed it with the expectation that CI tests will run before merge and catch it if there's an issue but I guess they don't auto run for new contributors

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