cmd/gc: liveness: don't add fallthough edge after panic

update #7205

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/50730044

src/cmd/gc/plive.c

@@ -538,6 +538,7 @@ newcfg(Prog *firstp)
 			switch(bb->last->as) {
 			case AJMP:
 			case ARET:
+			case AUNDEF:
 				break;
 			default:
 				addedge(bb, bb->last->link->opt);