Auto merge of #142941 - compiler-errors:shallow-bail, r=lcnr

Shallowly bail from `coerce_unsized` more

We do a *lot* of coercion in HIR typeck. Most of the time we're just coercing a type to itself, but we always try `coerce_unsized` even if it's not necessary.

Let's avoid doing that by adding a fast path to `coerce_unsized`; see the comment in that function.

Author: bors