Currently, when converting an access to an array element from ast to
IR, we need to see if the array is an ir_dereference_variable, and if
so update the variable's max_array_access.

When we add support for unsized arrays in interface blocks, we'll also
need to account for cases where the array is an ir_dereference_record
and the record is an interface block.

To make this easier, move the update into its own function.

v2: Use an ordinary function in ast_array_index.cpp rather than a
virtual function in ir_rvalue.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>