Note [Do not strictify a DFun's parameter dictionaries]

The typechecker can tie recursive knots involving (non-recursive) DFuns, so
we must not strictify a DFun's parameter dictionaries (#22549).
T22549 has an example involving undecidable instances that <<loop>>s when we
strictify the DFun of, e.g., `$fEqSeqT`:

  Main.$fEqSeqT
    = \@m @a ($dEq :: Eq (m (ViewT m a))) ($dMonad :: Monad m) ->
        GHC.Classes.C:Eq @(SeqT m a) ($c== @m @a $dEq $dMonad)
                                     ($c/= @m @a $dEq $dMonad)

  Rec {
    $dEq_a = Main.$fEqSeqT @Identity @Int $dEq_b Main.$fMonadIdentity
    $dEq_b = ... $dEq_a ... <another strict context due to DFun>
  }

If we make `$fEqSeqT` strict in `$dEq`, we'll collapse the Rec group into a
giant, <<loop>>ing thunk.

To prevent that, we never strictify dictionary params when inside a DFun.
That is implemented by unsetting 'dmd_strict_dicts' when entering a DFun.

See also Note [Speculative evaluation] in GHC.CoreToStg.Prep which has a rather
similar example in #20836. We may never speculate *arguments* of (recursive)
DFun calls, likewise we should not mark *formal parameters* of recursive DFuns
as strict.