interp: fix and refactor typeAssertStatus in

typeAssertStatus deals with the 3rd form of type assertion ("_, ok"), for
when one does not care about the result of the assertion itself.
Some cases for it, which are already fixed for the two other forms of
type assertions, had not been fixed for this form yet.

Therefore, this change fixes such cases for this form, while integrating
typeAssertStatus to the same code path as for the two other forms.

_test/assert0.go

@@ -48,6 +48,14 @@ func main() {
 	bType := reflect.TypeOf(TestStruct{})
 	fmt.Println(bType.Implements(aType))
 
+	// not redundant with the above, because it goes through a slightly different code path.
+	if _, ok := t.(MyWriter); !ok {
+		fmt.Println("TestStruct does not implement MyWriter")
+		return
+	} else {
+		fmt.Println("TestStruct implements MyWriter")
+	}
+
 	t = 42
 	foo, ok := t.(MyWriter)
 	if !ok {
@@ -57,6 +65,12 @@ func main() {
 	}
 	_ = foo
 
+	if _, ok := t.(MyWriter); !ok {
+		fmt.Println("42 does not implement MyWriter")
+	} else {
+		fmt.Println("42 implements MyWriter")
+	}
+
 	var tt interface{}
 	tt = time.Nanosecond
 	var myD MyStringer
@@ -72,6 +86,12 @@ func main() {
 	dType := reflect.TypeOf(time.Nanosecond)
 	fmt.Println(dType.Implements(cType))
 
+	if _, ok := tt.(MyStringer); !ok {
+		fmt.Println("time.Nanosecond does not implement MyStringer")
+	} else {
+		fmt.Println("time.Nanosecond implements MyStringer")
+	}
+
 	tt = 42
 	bar, ok := tt.(MyStringer)
 	if !ok {
@@ -81,6 +101,11 @@ func main() {
 	}
 	_ = bar
 
+	if _, ok := tt.(MyStringer); !ok {
+		fmt.Println("42 does not implement MyStringer")
+	} else {
+		fmt.Println("42 implements MyStringer")
+	}
 }
 
 // Output:
@@ -88,9 +113,13 @@ func main() {
 // 11
 // 11
 // true
+// TestStruct implements MyWriter
+// 42 does not implement MyWriter
 // 42 does not implement MyWriter
 // time.Nanosecond implements MyStringer
 // 1ns
 // 1ns
 // true
+// time.Nanosecond implements MyStringer
+// 42 does not implement MyStringer
 // 42 does not implement MyStringer