cmd/gc: don't require that slice index constants be small ints
cmd/gc: don't require that slice index constants be small ints

The test for this is test/index.go, which is not run by
default.

R=remyoudompheng, rsc
CC=golang-dev
http://codereview.appspot.com/6812089
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go