compiler: identify array index expressions in lvalue context

compiler: identify array index expressions in lvalue context

    Update the Mark_lvalue_varexprs visitor class (used in
    Assignment_statement::do_get_backend) to identify
    Array_index_expression's that appear on the left hand side of
    assignments, then pass this context info on when invoking
    Array_type::get_value_pointer.

    Reviewed-on: https://go-review.googlesource.com/44170

From-SVN: r248528