Mark Unit unspillable if it is merged with unspillable Unit

#KT-42004: Fixed

compiler/testData/codegen/box/coroutines/varSpilling/safeCallElvis.kt

@@ -0,0 +1,32 @@
+// WITH_RUNTIME
+
+import kotlin.coroutines.*
+
+var res: String = "FAIL"
+
+class Log {
+    fun error(message: Any?) {
+        res = message as String
+    }
+}
+
+private val log = Log()
+
+class C {
+    fun method() {}
+}
+
+fun <T : Any> df(t: T, r: suspend (T) -> Unit) {
+    r.startCoroutine(t, Continuation(EmptyCoroutineContext) {})
+}
+
+fun foo(s: String, c: C?) {
+    df(s) {
+        c?.method() ?: log.error(it)
+    }
+}
+
+fun box(): String {
+    foo("OK", null)
+    return res
+}