Avoid a loop in deep subsumption

sheafrequested to merge
sheaf/ghc:T26823 into master

Ticket: #26823 (closed)

This MR ensures we only unify after we recur in the deep subsumption code in the FunTy vs non-FunTy case of GHC.Tc.Utils.Unify.tc_sub_type_deep, to avoid falling into an infinite loop.

See the new Wrinkle [Avoiding a loop in tc_sub_type_deep] in Note [FunTy vs non-FunTy case in tc_sub_type_deep] in GHC.Tc.Utils.Unify.

Edited by sheaf

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