cmd/compile/internal/gc: cleaning; use range when appropriate

cmd/compile/internal/gc: cleaning; use range when appropriate

Made use of range statement in for loops.
Cleaning along the way:
-remove unnecessary variable declarations
-rename variables
-remove dead code

This change passes go build -toolexec 'toolstash -cmp' -a std.

Change-Id: Ife8c2a98482a81ba91f5bbb65142d9f3dc46d6ee
Reviewed-on: https://go-review.googlesource.com/14379
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>