Text-adjacent HTML comments cause markup generation to fail

[rkirsling]

Identified in tc39/ecma262#1589.

Normal case:

<p>
  Greetings.
</p>
<!-- comment -->
<p>The value *42* is my favorite.</p>

→

<p>
  Greetings.
  
</p>
<!-- comment -->
<p>The value <emu-val>42</emu-val> is my favorite.</p>

Problem case:

<p>
  Greetings.
  <!-- comment -->
</p>
<p>The value *42* is my favorite.</p>

→

<p>
  Greetings.
  
  <!-- comment -->
</p>
<p>The value *42* is my favorite.</p>

[bterlson]

Iiiiinteresting. I'll investigate.