Added phase RETURNS_INSERTION

Moved explicit insertion of return statement from code generator to a lowering

backend.native/tests/codegen/coroutines/degenerate1.kt

@@ -0,0 +1,22 @@
+import kotlin.coroutines.experimental.*
+import kotlin.coroutines.experimental.intrinsics.*
+
+open class EmptyContinuation(override val context: CoroutineContext = EmptyCoroutineContext) : Continuation<Any?> {
+    companion object : EmptyContinuation()
+    override fun resume(value: Any?) {}
+    override fun resumeWithException(exception: Throwable) { throw exception }
+}
+
+suspend fun s1() {
+    println("s1")
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun main(args: Array<String>) {
+    builder {
+        s1()
+    }
+}

backend.native/tests/codegen/coroutines/degenerate2.kt

@@ -0,0 +1,29 @@
+import kotlin.coroutines.experimental.*
+import kotlin.coroutines.experimental.intrinsics.*
+
+open class EmptyContinuation(override val context: CoroutineContext = EmptyCoroutineContext) : Continuation<Any?> {
+    companion object : EmptyContinuation()
+    override fun resume(value: Any?) {}
+    override fun resumeWithException(exception: Throwable) { throw exception }
+}
+
+suspend fun s1(): Unit = suspendCoroutineOrReturn { x ->
+    println("s1")
+    x.resume(Unit)
+    COROUTINE_SUSPENDED
+}
+
+suspend fun s2() {
+    println("s2")
+    s1()
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun main(args: Array<String>) {
+    builder {
+        s2()
+    }
+}