Note [Cast swizzling on rule LHSs]

In the LHS of a RULE we may have
       (\x. blah |> CoVar cv)
where `cv` is a coercion variable.  Critically, we really only want
coercion /variables/, not general coercions, on the LHS of a RULE.  So
we don't want to swizzle this to
      (\x. blah) |> (Refl xty `FunCo` CoVar cv)
So we switch off cast swizzling in updModeForRules.