As of March 2017 using escaped pipes is much easier:
\|
See other answers.
If you remove the backticks (`), using the |
hack works
a | r
------------|-----
`a += x;` | r1
a |= y; | r2
and produces the following output
Alternatively, you can replace the backticks (`) with a <code></code>
markup which fixes the issues more nicely by preserving the rendering
a | r
------------|-----
`a += x;` | r1
<code>a |= y;</code> | r2
generating the following output