[release-branch.go1.1] cmd/gc: do not corrupt init() with initializers of _ in closures.

««« CL 9952043 / c42a7c218440
cmd/gc: do not corrupt init() with initializers of _ in closures.

Fixes #5607.

R=golang-dev, daniel.morsing, r, dsymonds
CC=golang-dev
https://golang.org/cl/9952043
»»»

R=daniel.morsing, dsymonds, r, remyoudompheng
CC=golang-dev
https://golang.org/cl/9895044