cmd/compile/internal/gc: minor cleanup of slicing

cmd/compile/internal/gc: minor cleanup of slicing

Tidy the code up a little bit to move variable definitions closer
to uses, prefer early return to else branches and some other minor
tweaks.

I'd like to make some more changes to this code in the near future
and this CL should make those changes cleaner.

Change-Id: Ie7d7f2e4bb1e670347941e255c9cdc1703282db5
Reviewed-on: https://go-review.googlesource.com/c/go/+/170120
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>