cmd/gc: fix condition for fast pathed interface conversions

For some reason the current conditions require the type to be "uintptr-shaped".
This cuts off structs and arrays with a pointer.
isdirectiface and width==widthptr is sufficient condition to enable the fast paths.

Change-Id: I11842531e7941365413606cfd6c34c202aa14786
Reviewed-on: https://go-review.googlesource.com/3414Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/walk.c

@@ -883,8 +883,8 @@ walkexpr(Node **np, NodeList **init)
 	case OCONVIFACE:
 		walkexpr(&n->left, init);
 
-		// Optimize convT2E as a two-word copy when T is uintptr-shaped.
-		if(isnilinter(n->type) && isdirectiface(n->left->type) && n->left->type->width == widthptr && isint[simsimtype(n->left->type)]) {
+		// Optimize convT2E as a two-word copy when T is pointer-shaped.
+		if(isnilinter(n->type) && isdirectiface(n->left->type)) {
 			l = nod(OEFACE, typename(n->left->type), n->left);
 			l->type = n->type;
 			l->typecheck = n->typecheck;
@@ -927,7 +927,7 @@ walkexpr(Node **np, NodeList **init)
 			l->addable = 1;
 			ll = list(ll, l);
 
-			if(isdirectiface(n->left->type) && n->left->type->width == widthptr && isint[simsimtype(n->left->type)]) {
+			if(isdirectiface(n->left->type)) {
 				/* For pointer types, we can make a special form of optimization
 				 *
 				 * These statements are put onto the expression init list: