cmd/gc: Don't claim type assertion would help when it wont.

Fixes #3465.

R=golang-dev, rsc, remyoudompheng, iant
CC=golang-dev
https://golang.org/cl/6448097

src/cmd/gc/subr.c

@@ -1219,7 +1219,7 @@ assignop(Type *src, Type *dst, char **why)
 		return 0;
 	}
 	if(src->etype == TINTER && dst->etype != TBLANK) {
-		if(why != nil)
+		if(why != nil && implements(dst, src, &missing, &have, &ptr))
 			*why = ": need type assertion";
 		return 0;
 	}

test/interface/explicit.go

@@ -40,7 +40,7 @@ func main() {
 	// because i has an extra method
 	// that t does not, so i cannot contain a t.
 	i = t // ERROR "incompatible|missing M method"
-	t = i // ERROR "incompatible|need type assertion"
+	t = i // ERROR "incompatible|assignment$"
 
 	i = i2 // ok
 	i2 = i // ERROR "incompatible|missing N method"