aarch64: LD3/LD4 post-modify costs for struct modes

The LD3/ST3 and LD4/ST4 address cost code had no test coverage (oops).
This patch fixes that and updates it for the new structure modes.
The test only covers Advanced SIMD because SVE doesn't have
post-increment forms.

gcc/
	* config/aarch64/aarch64.c (aarch64_ldn_stn_vectors): New function.
	(aarch64_address_cost): Use it instead of testing for CImode and
	XImode directly.

gcc/testsuite/
	* gcc.target/aarch64/neoverse_v1_1.c: New test.