lcov over make check showed

 98      4699997 : sal_Int32 rtl_ustr_indexOfAscii_WithLength(
 99              :     sal_Unicode const * str, sal_Int32 len,
 100             :     char const * subStr, sal_Int32 subLen) SAL_THROW_EXTERN_C()
 101             : {
 102     4699997 :     assert(len >= 0);
 103     4699997 :     assert(subLen >= 0);
 104     4699997 :     if (subLen > 0 && subLen <= len) {
 105             :         sal_Int32 i;
 106    54014537 :         for (i = 0; i <= len - subLen; ++i) {
 107    51036513 :             if (rtl_ustr_asciil_reverseEquals_WithLength(
 108    51036523 :                     str + i, subStr, subLen))
 109             :             {
 110      205482 :                 return i;
 111             :             }
 112             :         }
 113             :     }
 114     4494505 :     return -1;
 115             : }

so
1/ in 95% of the cases the result is not-found.. _that_ is the hot path
2/ we are calling rtl_ustr_asciil_reverseEquals_WithLength close to 11 times
   per call.. (average ~ len - subLen, due to the high miss ratio)

so let's first search for the first byte of the substring
to optimize the 'miss' case, which is the most common one.

Change-Id: I20ef0821db2ff0db5935dd562844a947a14aff64
Reviewed-on: https://gerrit.libreoffice.org/16763Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>