cgen: optimize array op assign (#7178)

vlib/builtin/array_test.v

@@ -796,6 +796,57 @@ fn test_in_struct() {
 	assert baz.bar[0] == 3
 }
 
+[direct_array_access]
+fn test_direct_modification() {
+	mut foo := [2, 0, 5]
+	foo[1] = 3
+	foo[0] *= 7
+	foo[1]--
+	foo[2] -= 2
+	assert foo[0] == 14
+	assert foo[1] == 2
+	assert foo[2] == 3
+}
+
+fn test_shared_modification() {
+	shared foo := &[2, 0, 5]
+	lock foo {
+		unsafe {
+			foo[1] = 3
+			foo[0] *= 7
+			foo[1]--
+			foo[2] -= 2
+		}
+	}
+	rlock foo {
+		unsafe {
+			assert foo[0] == 14
+			assert foo[1] == 2
+			assert foo[2] == 3
+		}
+	}
+}
+
+[direct_array_access]
+fn test_shared_direct_modification() {
+	shared foo := &[2, 0, 5]
+	lock foo {
+		unsafe {
+			foo[1] = 3
+			foo[0] *= 7
+			foo[1]--
+			foo[2] -= 2
+		}
+	}
+	rlock foo {
+		unsafe {
+			assert foo[0] == 14
+			assert foo[1] == 2
+			assert foo[2] == 3
+		}
+	}
+}
+
 fn test_bools() {
 	println('test b')
 	mut a := [true, false]