compiler: merge runtime.typecodeID and runtime.typeInInterface

This distinction was useful before when reflect wasn't properly
supported. Back then it made sense to only include method sets that were
actually used in an interface. But now that it is possible to get to
other values (for example, by extracting fields from structs) and it is
possible to turn them back into interfaces, it is necessary to preserve
all method sets that can possibly be used in the program in a type
assert, interface assert or interface method call.

In the future, this logic will need to be revisited again when
reflect.New or reflect.Zero gets implemented.

Code size increases a bit in some cases, but usually in a very limited
way (except for one outlier in the drivers smoke tests). The next commit
will improve the situation significantly.

testdata/reflect.go

@@ -265,6 +265,19 @@ func main() {
 		println("type assertion failed (but should succeed)")
 	}
 
+	// Test type that is not referenced at all: not when creating the
+	// reflect.Value (except through the field) and not with a type assert.
+	// Previously this would result in a type assert failure because the Int()
+	// method wasn't picked up.
+	v = reflect.ValueOf(struct {
+		X totallyUnreferencedType
+	}{})
+	if v.Field(0).Interface().(interface {
+		Int() int
+	}).Int() != 42 {
+		println("could not call method on totally unreferenced type")
+	}
+
 	if reflect.TypeOf(new(myint)) != reflect.PtrTo(reflect.TypeOf(myint(0))) {
 		println("PtrTo failed for type myint")
 	}
@@ -363,6 +376,12 @@ func assertSize(ok bool, typ string) {
 
 type unreferencedType int
 
+type totallyUnreferencedType int
+
+func (totallyUnreferencedType) Int() int {
+	return 42
+}
+
 func TestStructTag() {
 	type S struct {
 		F string `species:"gopher" color:"blue"`