Fix evaluation order with object spread (#11412)

* Fix object spread runtime semantics

* limit this deoptimization

* optimize empty objects

* declare variables

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread/assignment/input.js

@@ -1,3 +1,7 @@
+var x;
+var y;
+var z;
+
 z = { x, ...y };
 
 z = { x, w: { ...y } };

packages/babel-plugin-proposal-object-rest-spread/test/fixtures/object-spread/assignment/output.js

@@ -4,6 +4,9 @@ function _objectSpread(target) { for (var i = 1; i < arguments.length; i++) { va
 
 function _defineProperty(obj, key, value) { if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
 
+var x;
+var y;
+var z;
 z = _objectSpread({
   x
 }, y);