cmd/compile: better variable names in internal/syntax/branches.go

Follow-up on https://go-review.googlesource.com/#/c/39998/.

Change-Id: I97f8e31ca923685198984ad64f952d6dc8208edf
Reviewed-on: https://go-review.googlesource.com/40982


Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>