aarch64: Fix target alignment for SVE [PR98119]

aarch64: Fix target alignment for SVE [PR98119]

The vectoriser supports peeling for alignment using predication:
we move back to the previous aligned boundary and make the skipped
elements inactive in the first loop iteration.  As it happens,
the costs for existing CPUs give an equal cost to aligned and
unaligned accesses, so this feature is rarely used.

However, the PR shows that when the feature was forced on, we were
still trying to align to a full-vector boundary even when using
partial vectors.

gcc/
	PR target/98119
	* config/aarch64/aarch64.c
	(aarch64_vectorize_preferred_vector_alignment): Query the size
	of the provided SVE vector; do not assume that all SVE vectors
	have the same size.

gcc/testsuite/
	PR target/98119
	* gcc.target/aarch64/sve/pr98119.c: New test.