slow validate failure: T15725 In scope set panic optasm

Reproduce with

hadrian/build test --freeze1  --docs=none --flavour=slow-validate --test-speed=slow --only="T15725"

ghc: panic! (the 'impossible' happened)
  GHC version 9.5.20220805:
	ASSERT failed!
  in_scope InScope {}
  tenv [aRk :-> (From1 x_aRG |> D:R:Rep1Identity[0] <a_aRF>_N)]
  tenvFVs {a_aRF, x_aRG}
  cenv []
  cenvFVs {}
  tys [x_aRG]
  cos []
  Call stack:
      CallStack (from HasCallStack):
        assertPpr, called at compiler/GHC/Core/TyCo/Subst.hs:683:5 in ghc:GHC.Core.TyCo.Subst
        checkValidSubst, called at compiler/GHC/Core/TyCo/Subst.hs:715:29 in ghc:GHC.Core.TyCo.Subst
        substTy, called at compiler/GHC/Core/TyCo/Subst.hs:248:43 in ghc:GHC.Core.TyCo.Subst

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


*** unexpected failure for T15725(optasm)