Refine 'handleResult' calls generation within coroutines

Before this change everything works just fine for 'handleResult' methods
accepting non-Unit parameters

For other cases the same coercion-to-unit strategy is in plain lambdas:
- if last statement is not Unit type, execute it, pop from the stack, then put Unit instance
- for 'return@label' (no expression) just put Unit on the stack

 #KT-13531 Fixed

compiler/testData/codegen/box/coroutines/coercionToUnit.kt

@@ -0,0 +1,46 @@
+class Controller {
+    var result = "fail"
+    operator fun handleResult(u: Unit, c: Continuation<Nothing>) {
+        result = "OK"
+    }
+
+    suspend fun <T> await(t: T, c: Continuation<T>) {
+        c.resume(t)
+    }
+}
+
+fun builder(coroutine c: Controller.() -> Continuation<Unit>): String {
+    val controller = Controller()
+    c(controller).resume(Unit)
+    return controller.result
+}
+
+var TRUE = true
+var FALSE = false
+fun box(): String {
+    val r1 = builder { await(Unit) }
+    if (r1 != "OK") return "fail 1"
+
+    val r2 = builder {
+        if (await(1) != 1) throw RuntimeException("fail1")
+
+        if (TRUE) return@builder
+    }
+    if (r2 != "OK") return "fail 2"
+
+    val r3 = builder {
+        if (await(1) != 1) throw RuntimeException("fail2")
+
+        if (FALSE) return@builder
+    }
+    if (r3 != "OK") return "fail 3"
+
+    val r4 = builder {
+        if (await(1) != 1) throw RuntimeException("fail3")
+
+        return@builder
+    }
+    if (r4 != "OK") return "fail 4"
+
+    return builder { await(1) }
+}