gen: fix evaluating the receiver of array methods (.map(), .filter(), etc) more than once (#7130)

vlib/v/tests/array_methods_test.v

@@ -0,0 +1,20 @@
+struct Counter {
+mut:
+	val int
+}
+
+// if this is called more than once, the test'll fail
+fn (mut c Counter) new_arr(msg string) []int {
+	if c.val > 0 { panic(msg) }
+	c.val++
+	return [1, 3, 2]
+}
+
+fn test_array_eval_count() {
+	// `new_arr()` should only be evaluated once, not on every iteration
+	mut a1 := Counter{}
+	assert a1.new_arr('map() failed').map(it * 2) == [2, 6, 4]
+
+	mut a2 := Counter{}
+	assert a2.new_arr('filter() failed').filter(it < 3) == [1, 2]
+}