[opt] Hoist comparisons across selects when it is cheap to do so

[cdleary]

Add a SelectSimplificationPass rewrite to push CompareOp through sel / priority_sel / one_hot_sel when comparing against a literal and the select-like node has ≤1 non-literal arm (including default) plus a single-user guard.

e.g.

eq(priority_sel(s, cases=[a, b, c], default=d), t)
  -> priority_sel(s,
                  cases=[eq(a, t), eq(b, t), eq(c, t)],
                  default=eq(d, t))

The fact we only do it when (at most) one case is non-constant means it should be more area-preserving to push this through. (It does suggest maybe we want some knob like --speculation_level=N similar to how we have --opt_level=N in the future though.)

For one_hot_sel, apply only when the selector is provably exactly-one-hot (IsKnownOneHotSelector), including common “one-hot by construction” patterns.

Extend GenericSelect with a small helper to rebuild the same select-like op with new arms (MakeSelectLikeWithNewArms).

Add query_engine_utils (+ tests) and new select_simplification_pass tests covering prio/sel/onehot, non-commutative operand ordering, and all-literal arms.

[meheff]

Nice!