x86: Fix *vec_dupv4hi constraints [PR94942]

x86: Fix *vec_dupv4hi constraints [PR94942]

This insn and split splits into HI->V?HImode broadcast for avx2 and later,
but either the operands need to be %xmm0-%xmm15 (i.e. VEX encoded insn), or
the insn needs both AVX512BW and AVX512VL.
Now, Yv constraint is v for AVX512VL and x otherwise, so for -mavx512vl -mno-avx512bw
we ICE if we end up with a %xmm16+ register from RA.
Yw constraint is v for AVX512VL and AVX512BW and nothing otherwise, so
in this pattern we actually need xYw.

2020-05-05  Jakub Jelinek  <jakub@redhat.com>

	PR target/94942
	* config/i386/mmx.md (*vec_dupv4hi): Use xYw constraints instead of Yv.

	* gcc.target/i386/pr94942.c: New test.