go/types: fix empty interface optimization (minor performance bug)

The tests checking for empty interfaces so that they can be fast-
tracked in the code actually didn't test the right field and the
fast track code never executed. Doing it now.

Change-Id: I58b2951efb3fb40b3366874c79fd653591ae0e99
Reviewed-on: https://go-review.googlesource.com/97519Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/interfaces.go

@@ -186,7 +186,7 @@ func (check *Checker) infoFromTypeLit(scope *Scope, iface *ast.InterfaceType, tn
 		check.interfaces[tname] = nil // computation started but not complete
 	}
 
-	if iface.Methods == nil {
+	if iface.Methods.List == nil {
 		// fast track for empty interface
 		info = &emptyIfaceInfo
 	} else {

src/go/types/typexpr.go

@@ -473,7 +473,7 @@ func (check *Checker) declareInSet(oset *objset, pos token.Pos, obj Object) bool
 
 func (check *Checker) interfaceType(ityp *Interface, iface *ast.InterfaceType, def *Named, path []*TypeName) {
 	// fast-track empty interface
-	if iface.Methods == nil {
+	if iface.Methods.List == nil {
 		ityp.allMethods = markComplete
 		return
 	}