[aarch64] check for non-NULL vectype in aarch64_vector_costs::add_stmt_cost

[aarch64] check for non-NULL vectype in aarch64_vector_costs::add_stmt_cost

With a patch still in development we get NULL STMT_VINFO_VECTYPE.
One side-effect is that during scalar stmt testing we no longer
pass a vectype.  The following adjusts aarch64_vector_costs::add_stmt_cost
to check for a non-NULL vectype before accessing it, like all the
code surrounding it.  The other fix possibility would have been
to re-orderr the check with the vect_mem_access_type one, but that
one is not going to exist during scalar code costing either in the
future.

	* config/aarch64/aarch64.cc (aarch64_vector_costs::add_stmt_cost):
	Check vectype is non-NULL before accessing it.