fix bug in IR regarding type aliases

testdata/alias.go

@@ -0,0 +1,38 @@
+package main
+
+type x struct{}
+
+func (x x) name() string {
+	return "x"
+}
+
+type y = x
+
+type a struct {
+	n int
+}
+
+func (a a) fruit() string {
+	return "apple"
+}
+
+type b = a
+
+type fruit interface {
+	fruit() string
+}
+
+type f = fruit
+
+func main() {
+	// test a basic alias
+	println(y{}.name())
+
+	// test using a type alias value as an interface
+	var v f = b{}
+	println(v.fruit())
+
+	// test comparing an alias interface with the referred-to type
+	println(a{} == b{})
+	println(a{2} == b{3})
+}