Refine returns codegen for suspend functions

This change should make the logic a bit more simple.
For all suspend functions/coroutines treat them in expression codegen
like they return boxed version of the original type.

Everything works fine then, except Unit type functions:
their bodies must be generated just like they're VOID and then load
Unit on stack manually.

compiler/testData/codegen/box/coroutines/unitTypeReturn/coroutineNonLocalReturn.kt

@@ -0,0 +1,46 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import kotlin.coroutines.*
+
+suspend fun suspendHere(): String = CoroutineIntrinsics.suspendCoroutineOrReturn { x ->
+    x.resume("OK")
+    CoroutineIntrinsics.SUSPENDED
+}
+
+fun builder(c: suspend () -> Unit) {
+    var wasResumeCalled = false
+    c.startCoroutine(object : Continuation<Unit> {
+        override fun resume(value: Unit) {
+            wasResumeCalled = true
+        }
+
+        override fun resumeWithException(exception: Throwable) {
+
+        }
+    })
+
+    if (!wasResumeCalled) throw RuntimeException("fail 1")
+}
+
+fun box(): String {
+    var result = ""
+
+    builder {
+        run {
+            if (result == "") return@builder
+        }
+        suspendHere()
+        throw RuntimeException("fail 2")
+    }
+
+    result = "fail1"
+
+    builder {
+        run {
+            if (result == "") return@builder
+        }
+        result = suspendHere()
+    }
+
+    return result
+}

compiler/testData/codegen/box/coroutines/unitTypeReturn/coroutineReturn.kt

@@ -0,0 +1,42 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import kotlin.coroutines.*
+
+suspend fun suspendHere(): String = CoroutineIntrinsics.suspendCoroutineOrReturn { x ->
+    x.resume("OK")
+    CoroutineIntrinsics.SUSPENDED
+}
+
+fun builder(c: suspend () -> Unit) {
+    var wasResumeCalled = false
+    c.startCoroutine(object : Continuation<Unit> {
+        override fun resume(value: Unit) {
+            wasResumeCalled = true
+        }
+
+        override fun resumeWithException(exception: Throwable) {
+
+        }
+    })
+
+    if (!wasResumeCalled) throw RuntimeException("fail 1")
+}
+
+fun box(): String {
+    var result = ""
+
+    builder {
+        if (result == "") return@builder
+        suspendHere()
+        throw RuntimeException("fail 2")
+    }
+
+    result = "fail"
+
+    builder {
+        if (result == "") return@builder
+        result = suspendHere()
+    }
+
+    return result
+}

compiler/testData/codegen/box/coroutines/unitTypeReturn/suspendNonLocalReturn.kt

@@ -0,0 +1,34 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+// IGNORE_BACKEND: JS
+import kotlin.coroutines.*
+
+var result = "0"
+
+
+suspend fun suspendHere(x: Int): Unit {
+    run {
+        if (x == 0) return
+        if (x == 1) return@suspendHere
+    }
+
+    result = "OK"
+    return CoroutineIntrinsics.suspendCoroutineOrReturn { x ->
+        x.resume(Unit)
+        CoroutineIntrinsics.SUSPENDED
+    }
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun box(): String {
+    builder {
+        if (suspendHere(0) != Unit) throw RuntimeException("fail 1")
+        if (suspendHere(1) != Unit) throw RuntimeException("fail 2")
+        if (suspendHere(2) != Unit) throw RuntimeException("fail 3")
+    }
+
+    return result
+}

compiler/testData/codegen/box/coroutines/unitTypeReturn/suspendReturn.kt

@@ -0,0 +1,27 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+import kotlin.coroutines.*
+
+var result = "0"
+
+suspend fun suspendHere(x: Int): Unit {
+    if (x == 0) return
+    result = "OK"
+    return CoroutineIntrinsics.suspendCoroutineOrReturn { x ->
+        x.resume(Unit)
+        CoroutineIntrinsics.SUSPENDED
+    }
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun box(): String {
+    builder {
+        suspendHere(0)
+        suspendHere(1)
+    }
+
+    return result
+}