i386: Enable intrinsics that convert float and bf16 data to each other.

gcc/ChangeLog:

	* config/i386/avx512bf16intrin.h (_mm_cvtsbh_ss): Add new intrinsic.
	(_mm512_cvtpbh_ps): Likewise.
	(_mm512_maskz_cvtpbh_ps): Likewise.
	(_mm512_mask_cvtpbh_ps): Likewise.
	* config/i386/avx512bf16vlintrin.h (_mm_cvtness_sbh): Likewise.
	(_mm_cvtpbh_ps): Likewise.
	(_mm256_cvtpbh_ps): Likewise.
	(_mm_maskz_cvtpbh_ps): Likewise.
	(_mm256_maskz_cvtpbh_ps): Likewise.
	(_mm_mask_cvtpbh_ps): Likewise.
	(_mm256_mask_cvtpbh_ps): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx512bf16-cvtsbh2ss-1.c: New test.
	* gcc.target/i386/avx512bf16-vcvtpbh2ps-1.c: Ditto.
	* gcc.target/i386/avx512bf16vl-cvtness2sbh-1.c: Ditto.
	* gcc.target/i386/avx512bf16vl-vcvtpbh2ps-1.c: Ditto.