Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/boxingOptimization/kt5844.kt

@@ -0,0 +1,29 @@
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+fun test1() {
+    val u = when (true) {
+        true -> 42
+        else -> 1.0
+    }
+
+    assertEquals(42, u)
+}
+
+fun test2() {
+    val u = 1L.let {
+        when (it) {
+            is Long -> if (it.toLong() == 2L) it.toLong() else it * 2L // CompilationException
+            else -> it.toDouble()
+        }
+    }
+
+    assertEquals(2L, u)
+}
+
+fun box(): String {
+    test1()
+    test2()
+    return "OK"
+}