Fix zero-masking for vcvtps2ph when dest operand is memory.

Fix zero-masking for vcvtps2ph when dest operand is memory.

When dest is memory, zero-masking is not valid, only merging-masking is available,

2020-06-24  Hongtao Liu  <hongtao.liu@inte.com>

gcc/ChangeLog:
	PR target/95254
	* config/i386/sse.md (*vcvtps2ph_store<merge_mask_name>):
	Refine from *vcvtps2ph_store<mask_name>.
	(vcvtps2ph256<mask_name>): Refine constraint from vm to v.
	(<mask_codefor>avx512f_vcvtps2ph512<mask_name>): Ditto.
	(*vcvtps2ph256<merge_mask_name>): New define_insn.
	(*avx512f_vcvtps2ph512<merge_mask_name>): Ditto.
	* config/i386/subst.md (merge_mask): New define_subst.
	(merge_mask_name): New define_subst_attr.
	(merge_mask_operand3): Ditto.

gcc/testsuite/ChangeLog:
	* gcc.target/i386/avx512f-vcvtps2ph-pr95254.c: New test.
	* gcc.target/i386/avx512vl-vcvtps2ph-pr95254.c: Ditto.