Tune single-byte code involving tolower

Tune single-byte code involving tolower

* lib/mbmemcasecmp.c (mbmemcasecmp):
* lib/mbscasecmp.c (mbscasecmp):
* lib/mbscasestr.c (mbscasestr):
* lib/mbsncasecmp.c (mbsncasecmp):
* lib/mbspcasecmp.c (mbspcasecmp):
Avoid some unnecessary calls to tolower.  For example, if the two
single-byte characters are equal before downcasing there is no
need to call tolower on either character.