Allow tags in any part of regexp, not only on top-level concatenation.

Allow tags in any part of regexp, not only on top-level concatenation.

Fixed-length tag optimization applies only to top-level concatenation:
since we cannot predict which path lexer will choose, we cannot be
sure that any tags except top-level concatenation will ever be
initialized. If a tag may be uninitialized, we cannot fix other
tags on this one (we cannot even fix same-level tags relative to
each other, because fixed tags won't preserve the default value).