aarch64: Add bfloat16 vldn/vstn intrinsics

aarch64: Add bfloat16 vldn/vstn intrinsics

This patch adds the load/store bfloat16 intrinsics to the AArch64 back-end.
ACLE documents are at https://developer.arm.com/docs/101028/latest
ISA documents are at https://developer.arm.com/docs/ddi0596/latest

2020-02-25  Mihail Ionescu  <mihail.ionescu@arm.com>

gcc/
	* config/aarch64/aarch64-builtins.c (aarch64_scalar_builtin_types):
	Add simd_bf.
	(aarch64_init_simd_builtin_scalar_types): Register simd_bf.
	(VAR15, VAR16): New.
	* config/aarch64/iterators.md (VALLDIF): Enable for V4BF and V8BF.
	(VD): Enable for V4BF.
	(VDC): Likewise.
	(VQ): Enable for V8BF.
	(VQ2): Likewise.
	(VQ_NO2E): Likewise.
	(VDBL, Vdbl): Add V4BF.
	(V_INT_EQUIV, v_int_equiv): Add V4BF and V8BF.
	* config/aarch64/arm_neon.h (bfloat16x4x2_t): New typedef.
	(bfloat16x8x2_t): Likewise.
	(bfloat16x4x3_t): Likewise.
	(bfloat16x8x3_t): Likewise.
	(bfloat16x4x4_t): Likewise.
	(bfloat16x8x4_t): Likewise.
	(vcombine_bf16): New.
	(vld1_bf16, vld1_bf16_x2): New.
	(vld1_bf16_x3, vld1_bf16_x4): New.
	(vld1q_bf16, vld1q_bf16_x2): New.
	(vld1q_bf16_x3, vld1q_bf16_x4): New.
	(vld1_lane_bf16): New.
	(vld1q_lane_bf16): New.
	(vld1_dup_bf16): New.
	(vld1q_dup_bf16): New.
	(vld2_bf16): New.
	(vld2q_bf16): New.
	(vld2_dup_bf16): New.
	(vld2q_dup_bf16): New.
	(vld3_bf16): New.
	(vld3q_bf16): New.
	(vld3_dup_bf16): New.
	(vld3q_dup_bf16): New.
	(vld4_bf16): New.
	(vld4q_bf16): New.
	(vld4_dup_bf16): New.
	(vld4q_dup_bf16): New.
	(vst1_bf16, vst1_bf16_x2): New.
	(vst1_bf16_x3, vst1_bf16_x4): New.
	(vst1q_bf16, vst1q_bf16_x2): New.
	(vst1q_bf16_x3, vst1q_bf16_x4): New.
	(vst1_lane_bf16): New.
	(vst1q_lane_bf16): New.
	(vst2_bf16): New.
	(vst2q_bf16): New.
	(vst3_bf16): New.
	(vst3q_bf16): New.
	(vst4_bf16): New.
	(vst4q_bf16): New.

gcc/testsuite/
	* gcc.target/aarch64/advsimd-intrinsics/bf16_vstn.c: New test.
	* gcc.target/aarch64/advsimd-intrinsics/bf16_vldn.c: New test.