regcomp.c: Create better estimate of trie match lengths

regcomp.c: Create better estimate of trie match lengths

This commit improves the estimate of the length of a string that a trie
matches.  Before this, the estimate gave more false positives, and
required some workarounds which are no longer necessary, and future
commits will remove.

The ultimate answer is to know precisely what will be matched.  As noted
in the comments, this information is already largely available in a
global variable.  But more work there needs to be done to complete it,
and make it conveniently accessible.