[release-branch.go1.1] cmd/gc: fix pointer composite literals in exported if statements.

««« CL 10470043 / d04e6b0bcacf
cmd/gc: fix pointer composite literals in exported if statements.

Fixes #4230 (again).

R=rsc, golang-dev, r
CC=golang-dev
https://golang.org/cl/10470043
»»»

Update #5928

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11455045