aarch64: ACLE intrinsics for BFCVTN, BFCVTN2 and BFCVT

aarch64: ACLE intrinsics for BFCVTN, BFCVTN2 and BFCVT

This patch adds the Armv8.6-a ACLE intrinsics for bfcvtn, bfcvtn2 and
bfcvt as part of the BFloat16 extension.
(https://developer.arm.com/architectures/instruction-sets/simd-isas/neon/intrinsics)
The intrinsics are declared in arm_bf16.h and arm_neon.h and the RTL
patterns are defined in aarch64-simd.md.

2020-03-06  Delia Burduv  <delia.burduv@arm.com>

gcc/
	* config/aarch64/aarch64-simd-builtins.def
	(bfcvtn): New built-in function.
	(bfcvtn_q): New built-in function.
	(bfcvtn2): New built-in function.
	(bfcvt): New built-in function.
	* config/aarch64/aarch64-simd.md
	(aarch64_bfcvtn<q><mode>): New pattern.
	(aarch64_bfcvtn2v8bf): New pattern.
	(aarch64_bfcvtbf): New pattern.
	* config/aarch64/arm_bf16.h (float32_t): New typedef.
	(vcvth_bf16_f32): New intrinsic.
	* config/aarch64/arm_bf16.h (vcvt_bf16_f32): New intrinsic.
	(vcvtq_low_bf16_f32): New intrinsic.
	(vcvtq_high_bf16_f32): New intrinsic.
	* config/aarch64/iterators.md (V4SF_TO_BF): New mode iterator.
	(UNSPEC_BFCVTN): New UNSPEC.
	(UNSPEC_BFCVTN2): New UNSPEC.
	(UNSPEC_BFCVT): New UNSPEC.
	* config/arm/types.md (bf_cvt): New type.

gcc/testsuite/
	* gcc.target/aarch64/advsimd-intrinsics/bfcvt-compile.c: New test.
	* gcc.target/aarch64/advsimd-intrinsics/bfcvt-nobf16.c: New test.
	* gcc.target/aarch64/advsimd-intrinsics/bfcvt-nosimd.c: New test.
	* gcc.target/aarch64/advsimd-intrinsics/bfcvtnq2-untied.c: New test.