cmd/compile: use range construct

cmd/compile: use range construct

so the code is more readable.

Also use n[i] = val instead of n = append(n, val),
because this avoids a function call to append.

NOTE: compiles, but I had trouble running toolstash -cmp and need sleep
now.

@Ian this might save you some grunt work :-)

Change-Id: I2a4c70396c58905f7d5aabf83f3020f11dea0e89
Reviewed-on: https://go-review.googlesource.com/20430
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>