Improve memcmpeq for 512-bit vector with vpcmpeq + kortest.

Improve memcmpeq for 512-bit vector with vpcmpeq + kortest.

When 2 vectors are equal, kmask is allones and kortest will set CF,
else CF will be cleared.

So CF bit can be used to check for the result of the comparison.

Before:
        vmovdqu (%rsi), %ymm0
        vpxorq  (%rdi), %ymm0, %ymm0
        vptest  %ymm0, %ymm0
        jne     .L2
        vmovdqu 32(%rsi), %ymm0
        vpxorq  32(%rdi), %ymm0, %ymm0
        vptest  %ymm0, %ymm0
        je      .L5
.L2:
        movl    $1, %eax
        xorl    $1, %eax
        vzeroupper
        ret

After:
        vmovdqu64       (%rsi), %zmm0
        xorl    %eax, %eax
        vpcmpeqd        (%rdi), %zmm0, %k0
        kortestw        %k0, %k0
        setc    %al
        vzeroupper
        ret

gcc/ChangeLog:

	PR target/104610
	* config/i386/i386-expand.cc (ix86_expand_branch): Handle
	512-bit vector with vpcmpeq + kortest.
	* config/i386/i386.md (cbranchxi4): New expander.
	* config/i386/sse.md: (cbranch<mode>4): Extend to V16SImode
	and V8DImode.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr104610-2.c: New test.