cmd/gc: avoid infinite recursion on invalid recursive type

Fixes issue 8507.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://codereview.appspot.com/144560043
cmd/gc: print x++ (not x += 1) in errors about x++

Fixes issue 8311.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, r
https://codereview.appspot.com/146270043