Prevent double custom element reaction enqueuing via upgrades

As already discussed in the spec (as of #1365), it is sometimes possible
to enqueue two upgrade reactions, via a constructor that does things you
are not supposed to do. The fix in #1365 prevented upgrading twice.
However, it did not bail out early enough, and allowed
attributeChangedCallback and connectedCallback reactions to be enqueued
(redundantly) before the bail-out. This commit moves the bail-out logic
to the beginning of the algorithm, so as to avoid any such
double-enqueuing.

Fixes WICG/webcomponents#523.

source

@@ -66669,16 +66669,6 @@ fetch(articleURL)
   <var>element</var>, run the following steps:</p>
 
   <ol>
-   <li><p>For each <var>attribute</var> in <var>element</var>'s <span>attribute list</span>, in
-   order, <span>enqueue a custom element callback reaction</span> with <var>element</var>, callback
-   name "<code data-x="">attributeChangedCallback</code>", and an argument list containing
-   <var>attribute</var>'s local name, null, <var>attribute</var>'s value, and <var>attribute</var>'s
-   namespace.</p></li>
-
-   <li><p>If <var>element</var> is currently <span>in a shadow-including document</span>, then
-   <span>enqueue a custom element callback reaction</span> with <var>element</var>, callback name
-   "<code data-x="">connectedCallback</code>", and an empty argument list.</p></li>
-
    <li>
     <p>If <var>element</var> is <span data-x="concept-element-custom">custom</span>, abort these
     steps.</p>
@@ -66715,6 +66705,16 @@ customElements.define("x-foo", class extends HTMLElement {
     </div>
    </li>
 
+   <li><p>For each <var>attribute</var> in <var>element</var>'s <span>attribute list</span>, in
+   order, <span>enqueue a custom element callback reaction</span> with <var>element</var>, callback
+   name "<code data-x="">attributeChangedCallback</code>", and an argument list containing
+   <var>attribute</var>'s local name, null, <var>attribute</var>'s value, and <var>attribute</var>'s
+   namespace.</p></li>
+
+   <li><p>If <var>element</var> is currently <span>in a shadow-including document</span>, then
+   <span>enqueue a custom element callback reaction</span> with <var>element</var>, callback name
+   "<code data-x="">connectedCallback</code>", and an empty argument list.</p></li>
+
    <li><p>Add <var>element</var> to the end of <var>definition</var>'s <span
    data-x="concept-custom-element-definition-construction-stack">construction stack</span>.</p></li>