patch 9.1.1184: Unnecessary use of vim_tolower() in vim_strnicmp_asc()

patch 9.1.1184: Unnecessary use of vim_tolower() in vim_strnicmp_asc()

Problem:  Unnecessary use of vim_tolower() in vim_strnicmp_asc().
Solution: Use TOLOWER_ASC() instead (zeertzjq).

It was passing *s1 and *s2 to vim_tolower(). When char is signed, which
is the case on most platforms, c < 0x80 is always true, so it already
behaves the same as TOLOWER_ASC().

closes: #16826

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>