cmd/6g: restore magic multiply for /=, %=.
cmd/6g: restore magic multiply for /=, %=.

Also enables turning /= 2 in a right shift.

Part of issue 2230.

R=rsc
CC=golang-dev, remy
http://codereview.appspot.com/6012049
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go