…r tags in the rightmost compound.

Before this patch we were only optimizing the case of a single selector, which
is fine, but not enough to catch ones like .foo .bar or so.

This patch allows us to optimize classes and tags in the rightmost compound,
while keeping the current optimization for #id selectors.

Need to profile this, but code-wise should be ready for review.

Differential Revision: https://phabricator.services.mozilla.com/D9351