[GISel] Fix #77762: extend correct source registers in combiner helper rule...

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

Since we already know which register we want to extend, we don't have to
ask its defining MI about it

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Co-authored-by: Emil Tywoniak <Emil.Tywoniak@hightec-rt.com>