T12176 and others: ASSERT failed on PowerPC 64-bit

Summary

T12176 fails with:

ghc-stage2: panic! (the 'impossible' happened)
  (GHC version 8.9.20190330 for powerpc64-unknown-linux):
        ASSERT failed!
  in_scope InScope {k_aYp a_aYs}
  tenv [aWU :-> (a_aYs[sk:2] |> {co_aYt})]
  tenvFVs {k_aYp[tau:1], a_aYs[sk:2], co_aYt}
  cenv []
  cenvFVs {}
  tys [Proxy a_aWU[sk:1] -> X]
  cos []
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1169:37 in ghc:Outputable
        pprPanic, called at compiler/utils/Outputable.hs:1230:5 in ghc:Outputable
        assertPprPanic, called at compiler/types/TyCoRep.hs:3199:56 in ghc:TyCoRep
        checkValidSubst, called at compiler/types/TyCoRep.hs:3230:29 in ghc:TyCoRep
        substTy, called at compiler/typecheck/TcCanonical.hs:1018:53 in ghc:TcCanonical

Testcases T13910, break006, InstantiatedNamedWildcardsInConstraints, NamedExtraConstraintsWildcard, T15453, T14350, T14605, and T14904a fail with the same ASSERT.

The testcases pass on powerpc64le Linux.

Note: I am using a GHC with patched the workaround from #15411 (closed).

The issue might be related to #15411 (closed).

Environment

• GHC version used: 8.9.20190330

Optional:

• Operating System: Linux
• System Architecture: powerpc64 (big-endian)