Anchor texturize to shortcodes to improve regex efficiency.

For the 4.0 branch; see [30449] for trunk.

props miqrogroove.
see #29557 for segfault issues.

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git-svn-id: http://core.svn.wordpress.org/branches/4.0@30444 1a063a9b-81f0-0310-95a4-ce76da25c4cd