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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