[AArch64] Handle arguments and return types with partial SVE modes

[AArch64] Handle arguments and return types with partial SVE modes

Partial SVE modes can be picked up and used by the vector_size(N)
attribute.[*] This means that we need to cope with arguments and return
values with partial SVE modes, which previously triggered asserts like:

  /* Generic vectors that map to SVE modes with -msve-vector-bits=N are
     passed by reference, not by value.  */
  gcc_assert (!aarch64_sve_mode_p (mode));

The ABI for these types is fixed from pre-SVE days, and must in any case
be the same for all -msve-vector-bits=N values.  All we need to do is
ensure that the vectors are passed and returned in the traditional way.

[*] Advanced SIMD always wins for 64-bit and 128-bit vectors though.

2019-12-19  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_function_value_1): New function,
	split out from...
	(aarch64_function_value): ...here.  Handle partial SVE modes by
	pretending that they have the associated/traditional integer mode,
	then wrap the result in the real mode.
	(aarch64_layout_arg): Take an orig_mode argument and pass it to
	aarch64_function_arg_alignment.  Handle partial SVE modes analogously
	to aarch64_function_value.
	(aarch64_function_arg): Update call accordingly.
	(aarch64_function_arg_advance): Likewise.

gcc/testsuite/
	* gcc.target/aarch64/sve/pcs/gnu_vectors_3.c: New test.

From-SVN: r279571