aarch64: Use memcpy to copy structures in bfloat vst* intrinsics

aarch64: Use memcpy to copy structures in bfloat vst* intrinsics

Use __builtin_memcpy to copy vector structures instead of using a
union - or constructing a new opaque structure one vector at a time -
in each of the vst[234][q] and vst1[q]_x[234] bfloat Neon intrinsics
in arm_neon.h.

Add new code generation tests to verify that superfluous move
instructions are not generated for the vst[234]q or vst1q_x[234]
bfloat intrinsics.

gcc/ChangeLog:

2021-07-30  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/arm_neon.h (vst1_bf16_x2): Use
	__builtin_memcpy instead of constructing an additional
	__builtin_aarch64_simd_oi one vector at a time.
	(vst1q_bf16_x2): Likewise.
	(vst1_bf16_x3): Use __builtin_memcpy instead of constructing
	an additional __builtin_aarch64_simd_ci one vector at a time.
	(vst1q_bf16_x3): Likewise.
	(vst1_bf16_x4): Use __builtin_memcpy instead of a union.
	(vst1q_bf16_x4): Likewise.
	(vst2_bf16): Use __builtin_memcpy instead of constructing an
	additional __builtin_aarch64_simd_oi one vector at a time.
	(vst2q_bf16): Likewise.
	(vst3_bf16): Use __builtin_memcpy instead of constructing an
	additional __builtin_aarch64_simd_ci mode one vector at a
	time.
	(vst3q_bf16): Likewise.
	(vst4_bf16): Use __builtin_memcpy instead of constructing an
	additional __builtin_aarch64_simd_xi one vector at a time.
	(vst4q_bf16): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/vector_structure_intrinsics.c: Add new
	tests.