cmd/compile: avoid range over copy of array

Passes toostash-check.

Slightly reduce compiler binary size:

file    before    after     Δ       %
compile 21087288  21070776  -16512  -0.078%
total   131847020 131830508 -16512  -0.013%

file                      before    after     Δ       %
cmd/compile/internal/gc.a 9007472   8999640   -7832   -0.087%
total                     127117794 127109962 -7832   -0.006%

Change-Id: I4aadd68d0a7545770598bed9d3a4d05899b67b52
Reviewed-on: https://go-review.googlesource.com/c/go/+/205777


Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>