ubsan: ubsan_maybe_instrument_array_ref tweak

In <https://gcc.gnu.org/pipermail/gcc-patches/2023-March/613687.html>
we discussed that the copy_node in ubsan_maybe_instrument_array_ref
is redundant, but also that it'd be best to postpone the optimization
to GCC 14.  So I'm making that change now.

gcc/c-family/ChangeLog:

	* c-ubsan.cc (ubsan_maybe_instrument_array_ref): Don't copy_node.