Removed incorrect optimization

tests/programs/reassign.q

@@ -0,0 +1,6 @@
+main() {
+	x := 1
+	y := x + 1
+	x = 2
+	syscall(60, y)
+}

tests/programs_test.go

@@ -22,6 +22,7 @@ var programs = []struct {
 	{"chained-calls.q", "", 9},
 	{"nested-calls.q", "", 4},
 	{"return.q", "", 6},
+	{"reassign.q", "", 2},
 }
 
 func TestPrograms(t *testing.T) {