cmd/gc: replace x*8 by x<<3 etc
cmd/gc: replace x*8 by x<<3 etc

Fixes issue 4199.

R=ken2
CC=golang-dev
https://codereview.appspot.com/7322081
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go