aarch64: ACLE intrinsics bfmmla and bfmlal<b/t>

aarch64: ACLE intrinsics bfmmla and bfmlal<b/t>

This patch adds the ARMv8.6 ACLE intrinsics for bfmmla, bfmlalb and
bfmlalt as part of the BFloat16 extension.
(https://developer.arm.com/architectures/instruction-sets/simd-isas/neon/intrinsics)
The intrinsics are declared in arm_neon.h and the RTL patterns are
defined in aarch64-simd.md.  Two new tests are added to check assembler
output.

2020-02-06  Delia Burduv  <delia.burduv@arm.com>

gcc/
	* config/aarch64/aarch64-simd-builtins.def
	(bfmlaq): New built-in function.
	(bfmlalb): New built-in function.
	(bfmlalt): New built-in function.
	(bfmlalb_lane): New built-in function.
	(bfmlalt_lane): New built-in function.
	* config/aarch64/aarch64-simd.md
	(aarch64_bfmmlaqv4sf): New pattern.
	(aarch64_bfmlal<bt>v4sf): New pattern.
	(aarch64_bfmlal<bt>_lane<q>v4sf): New pattern.
	* config/aarch64/arm_neon.h (vbfmmlaq_f32): New intrinsic.
	(vbfmlalbq_f32): New intrinsic.
	(vbfmlaltq_f32): New intrinsic.
	(vbfmlalbq_lane_f32): New intrinsic.
	(vbfmlaltq_lane_f32): New intrinsic.
	(vbfmlalbq_laneq_f32): New intrinsic.
	(vbfmlaltq_laneq_f32): New intrinsic.
	* config/aarch64/iterators.md (BF_MLA): New int iterator.
	(bt): New int attribute.