[IR] Fix ToAryInstr semantics

* This instruction is only used to implicit convert non-array
  values to arrays to support multiple-assignment.

  The ToAryNode use of this is going to be deleted since the
  ToAryNode is dead code.

* Fixes a failing spec which can be replicated with the
  following snippet

class ToAryNil
  def to_ary; end
end

a, b = ToAryNil.new
puts "a: #{a}"