stg-lint fails on unlifted-type join point binding

While validating GHC with BuildFlavour=devel2 and -dstg-lint I found that the linter chokes when it finds a join point of unlifted type (like this one from the Binary module),

ghc-stage1: panic! (the 'impossible' happened)
  (GHC version 8.3.20170815 for x86_64-unknown-linux):
          *** Stg Lint ErrMsgs: in Stg2Stg ***
  <no location info>: warning:
       [RHS of $j_sDSL :: (# State# RealWorld, () #)]
      Let(rec) binder ‘$j_sDSL’ has unlifted type ‘(# State# RealWorld,
                                                      () #)’
      RHS: \r []
               src<compiler/utils/Binary.hs:516:9-17>
               src<compiler/utils/Binary.hs:(516,21)-(518,26)>
               src<compiler/utils/Binary.hs:260:1-45>
               src<compiler/utils/Binary.hs:(239,1)-(246,34)>
               case readIntArray# [ww_sDSF 0# w1_sDSJ] of {
...

If I'm not mistaken there is no problem with join points having unlifted type (see Note [CoreSyn let/app invariant]). We just need to update the STG linter appropriately.

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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