aarch64: Pass and return Neon vector-tuple types without a parallel

aarch64: Pass and return Neon vector-tuple types without a parallel

Neon vector-tuple types can be passed in registers on function call
and return - there is no need to generate a parallel rtx. This patch
adds cases to detect vector-tuple modes and generates an appropriate
register rtx.

This change greatly improves code generated when passing Neon vector-
tuple types between functions; many new test cases are added to
defend these improvements.

gcc/ChangeLog:

2021-10-07  Jonathan Wright  <jonathan.wright@arm.com>

	* config/aarch64/aarch64.c (aarch64_function_value): Generate
	a register rtx for Neon vector-tuple modes.
	(aarch64_layout_arg): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/vector_structure_intrinsics.c: New code
	generation tests.