cmd/gc: don't emit type instructions for nonexisting locals
cmd/gc: don't emit type instructions for nonexisting locals

If all locals are optimized away, the type instructions would stay in the instruction stream. Call fixautoused to scrub the output.

Fixes issue 4915.

R=rsc
CC=golang-dev
https://codereview.appspot.com/7385055
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go