Auto merge of #29960 - aturon:dismantle-traitref-subtyping, r=nikomatsakis
Trait references are always invariant, so all uses of subtyping between them are equivalent to using equality. Moreover, the overlap check was previously performed twice per impl pair, once in each direction. It is now performed only once, and internally uses the equality check. On glium, a crate that spends some time in coherence, this change sped up coherence checking by a few percent (not very significant). r? @nikomatsakis
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