cgen: fix enum in map (#9912)

vlib/builtin/map_test.v

@@ -617,6 +617,15 @@ type ColorAlias = Color
 fn test_alias_enum() {
 	mut m := map[ColorAlias]string{}
 	m[Color.red] = 'hi'
+	assert m[Color.red] == 'hi'
+}
+
+fn test_enum_in_map() {
+	mut m := map[Color]string{}
+	m[Color.red] = 'hi'
+	assert Color.red in m
+	assert Color.green !in m
+	assert Color.blue !in m
 }
 
 fn test_voidptr_keys() {