[dev.ssa] cmd/compile/ssa: stop compilation immediately on leading goto

There is clearly work to do to fix labels and gotos.
The compiler currently hangs on ken/label.go.

For the moment, stop the bleeding.

Fixes the build.

Change-Id: Ib68360d583cf53e1a8ca4acff50644b570382728
Reviewed-on: https://go-review.googlesource.com/11877Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/ssa.go

@@ -330,6 +330,7 @@ func (s *state) stmt(n *Node) {
 		}
 		if n.Op == OGOTO && s.curBlock == nil {
 			s.Unimplementedf("goto at start of function; see test/goto.go")
+			panic("stop compiling here, on pain of infinite loops")
 		}
 
 	case OAS, OASWB: