Allow trie use for /iaa matching

Allow trie use for /iaa matching

This adds code so that tries can be formed under /iaa, which formerly
weren't handled.  A problem occurs when the string contains the LATIN
SMALL LETTER SHARP S when the regex pattern is not UTF-8 encoded.  I
tried several ways to get this to work easily, but ended up deciding it
was too hard, to in this one situation, a new regnode is created to
prevent the trie code from even trying to turn it into a trie.