cmd/gc: Add tests for self-referential array types.
cmd/gc: Add tests for self-referential array types.

LGTM=gri, iant
R=gri, iant
CC=golang-codereviews
https://codereview.appspot.com/77050045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang