UnliftedNewtypesDifficultUnification broken due to simplified subsumption

UnliftedNewtypesDifficultUnification fails due to simplified subsumption patch (!2600 (merged)) in DEBUG-enabled compiler.

Compile failed (exit code 1) errors were:
ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.11.0.20200603:
	ASSERT failed! file compiler/GHC/Core/FamInstEnv.hs, line 651

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug


*** unexpected failure for UnliftedNewtypesDifficultUnification(normal)

I have marked this test as broken in the interest of getting !2600 (merged) merged.

As seen in Job #356282.

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