Missing priority select optimizations

[grebe]

I'm just jotting down some thoughts on priority select optimizations we could do but aren't doing yet, feel free to add more.

• If p_i is the ith selector bit and p_j->p_i for some i<j, case j will never be selected and we can drop it.
• If some contiguous subset of selector bits are mutually exclusive, those case arms can be hoisted out into a onehot select
• We use query engines to prove properties about selectors during codegen. Block conversion (esp. pipeline registers) can obscure the possible values of state elements, so a assume op (see #542) could be added to for relevant selector properties before scheduling+codegen.