https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78809
Inline strcmp with small constant strings

The design doc for PR78809 is at:
https://www.mail-archive.com/gcc@gcc.gnu.org/msg83822.html

this patch is for the third part of change of PR78809.

C. for strcmp (s1, s2), strncmp (s1, s2, n), and memcmp (s1, s2, n)
   if the result is NOT used to do simple equality test against zero, one of
"s1" or "s2" is a small constant string, n is a constant, and the Min value of
the length of the constant string and "n" is smaller than a predefined
threshold T,
   inline the call by a byte-to-byte comparision sequence to avoid calling
overhead.

adding test case strcmpopt_5.c into gcc.dg for part C of PR78809.
adding test case strcmpopt_6.c into gcc.dg to test the following case:
   When the specified length exceeds one of the arguments of the call to memcmp,
   the call to memcmp should NOT be inlined.

From-SVN: r262636