make ! support the equality/ordering operators

An expression such as `bottom == not_bottom` or `not_bottom == bottom`
already compiled, but this fixes the case where both sides are `bottom`.
auto merge of #9174 : thestinger/rust/bot, r=catamorphism

An expression such as `bottom == not_bottom` or `not_bottom == bottom`
already compiled, but this fixes the case where both sides are `bottom`.