Fix PathHoister hoisting before a same-scope variable declaration.

Seems we didn't have tests running for this very simple case.

Also fixes #5520

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/var/actual.js

@@ -0,0 +1,6 @@
+function fn(Component) {
+
+  var data = "prop";
+
+  return () => <Component prop={data} />;
+}

packages/babel-plugin-transform-react-constant-elements/test/fixtures/constant-elements/var/expected.js

@@ -0,0 +1,7 @@
+function fn(Component) {
+
+  var data = "prop",
+      _ref = <Component prop={data} />;
+
+  return () => _ref;
+}