cgen: auto initialize chan that are struct elements (#8541)

vlib/sync/struct_chan_init_test.v

@@ -0,0 +1,14 @@
+struct Abc {
+	ch chan int
+}
+
+fn f(st Abc) {
+	st.ch <- 47
+}
+
+fn test_chan_init() {
+	st := Abc{}
+	go f(st)
+	i := <-st.ch
+	assert i == 47
+}