[Arc] Fix folding of initialized StateOp

[fzi-hielscher]

When adding the initial value parameter to the StateOp I forgot to adapt its folder. This PR should fix that.

• Fold a permanently disabled state to its initial value (if present) instead of zero.
• Don't fold a state that is permanently reset if it has an explicit initial value.
• Don't attempt to constant fold a result-less StateOp to avoid an infinite loop.

[maerhart]

Thanks for the fix! And great catch with the 0 result values.

[fzi-hielscher]

Sorry, it just occurred to me that this is also incorrect if the initial value is time-variant (you'd think I knew better after the discussion in #7638 😅 ). We need to make sure that a 'dynamic' initial value is materialized in the arc.initial op and not in the actual clock tree.
I'll restrict the folding to constants for now...