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Allow multiple case branches to have a higher rank type

Simon Peyton Jones requested to merge wip/T18412 into master

As #18412 (closed) points out, it should be OK for multiple case alternatives to have a higher rank type, provided they are all the same.

This patch implements that change. It sweeps away GHC.Tc.Gen.Match.tauifyMultipleBranches, and friends, replacing it with an enhanced version of fillInferResult.

The basic change to fillInferResult is to permit the case in which another case alternative has already filled in the result; and in that case simply unify. It's very simple actually.

See the new Note [fillInferResult].

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