unsafePerformIO makes bottoming actions appear non-bottoming

In general GHC avoids inlining bottoming expressions since they are typically in cold-paths. However, while working on making runRW# simplify (#15127 (closed)) I noticed that PlainPanic.panic tends to be inlined often. For instance, in GHC.CmmToAsm.PPC.Regs we alone inlined PlainPanic.panic six times, each of which generating a 20-something term binding, each differing only in the error message string. For instance,

-- RHS size: {terms: 17, types: 41, coercions: 0, joins: 0/0}
GHC.CmmToAsm.PPC.Regs.regDotColor2
  :: GHC.Prim.State# GHC.Prim.RealWorld -> Outputable.SDoc
[GblId, Arity=1, Str=<L,U>, Cpr=b, Unf=OtherCon []]
GHC.CmmToAsm.PPC.Regs.regDotColor2
  = \ (eta_s8Vu [Occ=Once] :: GHC.Prim.State# GHC.Prim.RealWorld) ->
      case GHC.Prim.getCurrentCCS#
             @GHC.Base.String @GHC.Prim.RealWorld x1_r7VT eta_s8Vu
      of
      { (# s'_s8Vw [Occ=Once], addr_s8Vx [Occ=Once] #) ->
      case GHC.Stack.CCS.$wgo
             addr_s8Vx (GHC.Types.[] @[GHC.Types.Char]) s'_s8Vw
      of
      { (# ipv_s8Vz [Occ=Once], ipv1_s8VA [Occ=Once] #) ->
      case PlainPanic.panic1
             @GHC.Platform.Reg.Class.RegClass ipv_s8Vz ipv1_s8VA x1_r7VT
      of {
      }
      }
      }

The reason for this is likely that panic uses unsafeDupablePerformIO, which hides the bottoming nature of the IO action which it runs.

See comments on !3126 (closed)