Stg rewriter should create updatable closures

In !9874 (comment 518337) we noticed that the rewriteRhs function is creating ReEntrant closures, see the last line of this case:

rewriteRhs (_id, _tagSig) (StgRhsCon ccs con cn ticks args typ) = {-# SCC rewriteRhs_ #-} do
    -- pprTraceM "rewriteRhs" (ppr _id)

    -- Look up the nodes representing the constructor arguments.
    fieldInfos <- mapM isArgTagged args

    -- Filter out non-strict fields.
    let strictFields =
            getStrictConArgs con (zip args fieldInfos) :: [(StgArg,Bool)] -- (nth-argument, tagInfo)
    -- Filter out already tagged arguments.
    let needsEval = map fst . --get the actual argument
                        filter (not . snd) $ -- Keep untagged (False) elements.
                        strictFields :: [StgArg]
    let evalArgs = [v | StgVarArg v <- needsEval] :: [Id]

    if (null evalArgs)
        then return $! (StgRhsCon ccs con cn ticks args typ)
        else do
            --assert not (isTaggedSig tagSig)
            -- pprTraceM "CreatingSeqs for " $ ppr _id <+> ppr node_id

            -- At this point iff we have  possibly untagged arguments to strict fields
            -- we convert the RHS into a RhsClosure which will evaluate the arguments
            -- before allocating the constructor.
            let ty_stub = panic "mkSeqs shouldn't use the type arg"
            conExpr <- mkSeqs args evalArgs (\taggedArgs -> StgConApp con cn taggedArgs ty_stub)

            fvs <- fvArgs args
            -- lcls <- getFVs
            -- pprTraceM "RhsClosureConversion" (ppr (StgRhsClosure fvs ccs ReEntrant [] $! conExpr) $$ text "lcls:" <> ppr lcls)
            return $! (StgRhsClosure fvs ccs ReEntrant [] $! conExpr) typ

This means that all allocations from conExpr could be repeated many times, which is especially painful if conExpr is an infinite recursive data type. See for an example: !9874 (comment 517869).

If instead we mark this Updatable then the allocations in conExpr will only be performed once.

It would be nice to find a minimal reproducer for this to add to the test suite, but this rewriteRhs function is only run in some very specific situations which are hard to reproduce.