aarch64: Don't classify vector pairs as short vectors [PR103094]

In this PR we were wrongly classifying a pair of 8-byte vectors
as a 16-byte “short vector” (in the AAPCS64 sense).  As the
comment in the patch says, this stems from an old condition
in aarch64_short_vector_p that is too loose, but that would
be difficult to tighten now.

We can still do the right thing for the newly-added modes though,
since there are no backwards compatibility concerns there.

Co-authored-by: Tamar Christina <tamar.christina@arm.com>

gcc/
	PR target/103094
	* config/aarch64/aarch64.c (aarch64_short_vector_p): Return false
	for structure modes, rather than ignoring the type in that case.

gcc/testsuite/
	PR target/103094
	* gcc.target/aarch64/pr103094.c: New test.