aarch64: intrinsics extract half of bf16 vector

aarch64: intrinsics extract half of bf16 vector

This patch implements ACLE intrinsics vget_low_bf16 and vget_high_bf16
to extract lower or higher half from a bfloat16x8 vector. The
vget_high_bf16 is done by 'dup' instruction. The vget_low_bf16 is just
to return the lower half of a vector register. Tests include both big-
and little-endian cases.

gcc/ChangeLog:

2020-11-03  Dennis Zhang  <dennis.zhang@arm.com>

	* config/aarch64/aarch64-simd-builtins.def (vget_lo_half): New entry.
	(vget_hi_half): Likewise.
	* config/aarch64/aarch64-simd.md (aarch64_vget_lo_halfv8bf): New entry.
	(aarch64_vget_hi_halfv8bf): Likewise.
	* config/aarch64/arm_neon.h (vget_low_bf16): New intrinsic.
	(vget_high_bf16): Likewise.

gcc/testsuite/ChangeLog

	* gcc.target/aarch64/advsimd-intrinsics/bf16_get.c: New test.
	* gcc.target/aarch64/advsimd-intrinsics/bf16_get-be.c: New test.