Use is_utf8_invariant_string() more

Use is_utf8_invariant_string() more

Now that this function was changed to do word-at-a time searching in
commit e17544a60909ed9555c0dad7cd24afc40eb736e7, we can more quickly
find the first variant byte in a string, if any.  Given that a lot of
usage of Perl is on ASCII data, it makes sense to try this first before
any byte-at-a-time processing.

Since Perl can be used on things that are mostly non-ASCII, we give up
at the first such one, and process the rest of the string byte-by-byte.
Otherwise we could have a pipeline of finding the next variant quickly,
but this would only be faster if variants were rare, which I don't feel
we can be confident about, after finding at least one.