docs: document INTEL_DEBUG 'optimizer' envvar

Reviewed-by: Matt Turner <mattst88@gmail.com>

docs/envvars.html

@@ -153,6 +153,7 @@ See the <a href="xlibdriver.html">Xlib software driver page</a> for details.
    <li>no16 - suppress generation of 16-wide fragment shaders. useful for debugging broken shaders</li>
    <li>blorp - emit messages about the blorp operations (blits &amp; clears)</li>
    <li>nodualobj - suppress generation of dual-object geometry shader code</li>
+   <li>optimizer - dump shader assembly to files at each optimization pass and iteration that make progress</li>
 </ul>
 </ul>