cgen: fix for mut val and enable the test

vlib/v/tests/for_loops_test.v

@@ -56,12 +56,10 @@ fn test_for_char_in_map() {
 }
 
 fn test_mut_for() {
-	/*
 	mut vals := [1, 2, 3]
 	for mut val in vals {
 		(*val)++
 	}
 	assert vals == [2, 3, 4]
 	println(vals)
-	*/
 }