Merge boxWithStdlib testData into box, delete BoxWithStdlib test

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

@@ -0,0 +1,14 @@
+// WITH_RUNTIME
+
+import kotlin.test.assertEquals
+
+fun box(): String {
+    var result = 0
+    if (1 == 1) {
+        val x: Int? = 1
+        result += x!!
+    }
+
+    assertEquals(1, result)
+    return "OK"
+}