Eliminate verbose rewrite rules for ambiguous rewrite structures, resulting in...

Eliminate verbose rewrite rules for ambiguous rewrite structures, resulting in massive performance gains. props andy, otto42, duck_. Nice work everyone! see #16687

git-svn-id: https://core.svn.wordpress.org/trunk@18541 1a063a9b-81f0-0310-95a4-ce76da25c4cd