fix broken code produced for ominus
fix broken code produced for ominus

go/test: passes 89% (308/345)

R=rsc
APPROVED=rsc
DELTA=10  (5 added, 0 deleted, 5 changed)
OCL=36005
CL=36021
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go