5g: don't generate reg variables for direct-called functions

The test intended to skip direct calls when creating
registerization variables was testing p->to.type instead of
p->to.name, so it always failed, causing regopt to create
unnecessary variables for these names.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/169110043

src/cmd/5g/reg.c

@@ -199,7 +199,7 @@ regopt(Prog *firstp)
 		proginfo(&info, p);
 
 		// Avoid making variables for direct-called functions.
-		if(p->as == ABL && p->to.type == D_EXTERN)
+		if(p->as == ABL && p->to.name == D_EXTERN)
 			continue;
 
 		bit = mkvar(r, &p->from);