[AArch64] Strengthen aarch64_hard_regno_call_part_clobbered

[AArch64] Strengthen aarch64_hard_regno_call_part_clobbered

The aarch64_vector_pcs handling in aarch64_hard_regno_call_part_clobbered
checks whether the mode might be bigger than 16 bytes, since on SVE
targets the (non-SVE) vector PCS only guarantees that the low 16 bytes
are preserved.  But for multi-register modes, we should instead test
whether each single-register part might be bigger than 16 bytes.
(The size is always divided evenly between registers.)

The testcase uses XImode as an example where this helps.

2019-09-30  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_hard_regno_call_part_clobbered):
	For multi-registers modes, test how big each register part is.

gcc/testsuite/
	* gcc.target/aarch64/torture/simd-abi-8.c: New test.

From-SVN: r276305