FE: properly clear the deferred generator stack

Before this commit the stack wasn't cleared for read variable case.
#KT-47168 Fixed

compiler/testData/codegen/box/contracts/kt47168.kt

@@ -0,0 +1,16 @@
+// !USE_EXPERIMENTAL: kotlin.contracts.ExperimentalContracts
+// IGNORE_BACKEND: NATIVE
+import kotlin.contracts.*
+
+inline fun foo(x: () -> String, y: () -> String): String {
+    contract {
+        callsInPlace(x, InvocationKind.EXACTLY_ONCE)
+        callsInPlace(y, InvocationKind.EXACTLY_ONCE)
+    }
+    return x() + y()
+}
+
+fun box(): String {
+    val y = { "K" }
+    return foo({ "O" }, y)
+}