cgen: fix deadlock when returning/breaking in lock (#10079)

vlib/v/tests/break_in_lock_test.v

@@ -0,0 +1,53 @@
+import time
+
+struct AA {
+mut:
+	b string
+}
+
+const (
+	sleep_time = time.millisecond * 50
+)
+
+fn test_return_lock() {
+	shared s := AA{'3'}
+	go printer(shared s)
+	go fn (shared s AA) {
+		start := time.now()
+		for {
+			reader(shared s)
+			if time.now() - start > sleep_time {
+				exit(0)
+			}
+		}
+	}(shared s)
+	time.sleep(sleep_time * 2)
+	assert false
+}
+
+fn printer(shared s AA) {
+	start := time.now()
+	for {
+		lock s {
+			assert s.b in ['0', '1', '2', '3', '4', '5']
+		}
+		if time.now() - start > time.millisecond * 50 {
+			exit(0)
+		}
+	}
+}
+
+fn reader(shared s AA) {
+	mut i := 0
+	for {
+		i++
+		x := i.str()
+		lock s {
+			s.b = x
+			if s.b == '5' {
+				// this test checks if cgen unlocks the mutex here
+				break
+			}
+		}
+	}
+}