x86: Add cmpmemsi for -minline-all-stringops

x86: Add cmpmemsi for -minline-all-stringops

We used to expand memcmp to "repz cmpsb" via cmpstrnsi.  It was changed
by

commit 9b0f6f5e511ca512e4faeabc81d2fd3abad9b02f
Author: Nick Clifton <nickc@redhat.com>
Date:   Fri Aug 12 16:26:11 2011 +0000

    builtins.c (expand_builtin_memcmp): Do not use cmpstrnsi pattern.

            * builtins.c (expand_builtin_memcmp): Do not use cmpstrnsi
            pattern.
            * doc/md.texi (cmpstrn): Note that the comparison stops if both
            fetched bytes are zero.
            (cmpstr): Likewise.
            (cmpmem): Note that the comparison does not stop if both of the
            fetched bytes are zero.

Duplicate the cmpstrn pattern for cmpmem.  The only difference is that
the length argument of cmpmem is guaranteed to be less than or equal to
lengths of 2 memory areas.  Since "repz cmpsb" can be much slower than
memcmp function implemented with vector instruction, see

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43052

expand cmpmem to "repz cmpsb" only for -minline-all-stringops.

gcc/

	PR target/95151
	* config/i386/i386-expand.c (ix86_expand_cmpstrn_or_cmpmem): New
	function.
	* config/i386/i386-protos.h (ix86_expand_cmpstrn_or_cmpmem): New
	prototype.
	* config/i386/i386.md (cmpmemsi): New pattern.

gcc/testsuite/

	PR target/95151
	* gcc.target/i386/pr95151-1.c: New test.
	* gcc.target/i386/pr95151-2.c: Likewise.
	* gcc.target/i386/pr95151-3.c: Likewise.
	* gcc.target/i386/pr95151-4.c: Likewise.