gc: avoid saying same error 3 times

R=ken2
CC=golang-dev
https://golang.org/cl/4316051

src/cmd/gc/const.c

@@ -136,7 +136,6 @@ convlit1(Node **np, Type *t, int explicit)
 	case CTNIL:
 		switch(et) {
 		default:
-			yyerror("cannot use nil as %T", t);
 			n->type = T;
 			goto bad;
 

src/cmd/gc/subr.c

@@ -2053,13 +2053,16 @@ Node*
 assignconv(Node *n, Type *t, char *context)
 {
 	int op;
-	Node *r;
+	Node *r, *old;
 	char *why;
 	
 	if(n == N || n->type == T)
 		return n;
 
+	old = n;
+	old->diag++;  // silence errors about n; we'll issue one below
 	defaultlit(&n, t);
+	old->diag--;
 	if(t->etype == TBLANK)
 		return n;
 

test/ddd1.go

@@ -15,7 +15,7 @@ var (
 	_ = sum()
 	_ = sum(1.0, 2.0)
 	_ = sum(1.5)      // ERROR "integer"
-	_ = sum("hello")  // ERROR "convert|incompatible"
+	_ = sum("hello")  // ERROR "string.*as type int|incompatible"
 	_ = sum([]int{1}) // ERROR "slice literal.*as type int|incompatible"
 )