cmd/compile: internal compiler error: b3.Succs has duplicate block b3 #11593

on dev.ssa branch
go version devel +be2a3e2 Thu Jul 2 08:29:33 2015 +0000 linux/amd64

gc crashes on the following program:

package a
func f_ssa() {_:{_:_:}}

go.go:2: internal compiler error: b3.Succs has duplicate block b3