Don't lower vpcmpu to pcmpgt since the latter is for signed comparison.

r15-1737-gb06a108f0fbffe lower AVX512 kmask comparison to AVX2 ones,
but wrong lowered unsigned comparison to signed ones, for unsigned
comparison, only EQ/NEQ can be lowered.

The commit fix that.

gcc/ChangeLog:

	PR target/116940
	* config/i386/sse.md (*avx2_pcmp<mode>3_7): Change
	UNSPEC_PCMP_ITER to UNSPEC_PCMP.
	(*avx2_pcmp<mode>3_8): New pre_reload
	define_insn_and_splitter.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr116940.c: New test.