i386: Auto vectorize sdot_prod, usdot_prod with VNNI instruction.

i386: Auto vectorize sdot_prod, usdot_prod with VNNI instruction.

AVX512VNNI/AVXVNNI has vpdpwssd for HImode, vpdpbusd for QImode, so
Adjust HImode sdot_prod expander and add QImode usdot_prod expander
to enhance vectorization for dotprod.

gcc/ChangeLog:

	* config/i386/sse.md (VI2_AVX512VNNIBW): New mode iterator.
	(VI1_AVX512VNNI): Likewise.
	(SDOT_VPDP_SUF): New mode_attr.
	(VI1SI): Likewise.
	(vi1si): Likewise.
	(sdot_prod<mode>): Use VI2_AVX512F iterator, expand to
	vpdpwssd when VNNI targets available.
	(usdot_prod<mode>): New expander for vector QImode.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/vnni-auto-vectorize-1.c: New test.
	* gcc.target/i386/vnni-auto-vectorize-2.c: Ditto.