aarch64: Make sure the UF divides the VF [PR105254]

aarch64: Make sure the UF divides the VF [PR105254]

In this PR, we were trying to set the unroll factor to a value higher
than the minimum VF (or more specifically, to a value that doesn't
divide the VF).  I guess there are two approaches to this: let the
target pick any value it likes and make target-independent code pare
it back to something that makes sense, or require targets to supply
sensible values from the outset.  This patch goes for the latter
approach.

gcc/
	PR tree-optimization/105254
	* config/aarch64/aarch64.cc
	(aarch64_vector_costs::determine_suggested_unroll_factor): Take a
	loop_vec_info as argument.  Restrict the unroll factor to values
	that divide the VF.
	(aarch64_vector_costs::finish_cost): Update call accordingly.

gcc/testsuite/
	PR tree-optimization/105254
	* g++.dg/vect/pr105254.cc: New test.