Switch Reaching analysis to use Finset for more efficiency

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lean/Spa/Transformation/Licm.lean

@@ -65,10 +65,10 @@ def lookupDef (prog : Program) (vs : VariableValues (DefSet prog) prog)
     (k : String) : DefSet prog :=
   if h : FiniteMap.MemKey k vs then (FiniteMap.locate h).1 else ⊥
 
-/-- The AST node ids marked as definition sites in a `DefSet` (those mapped to
-`true`). With the AST-id-keyed lattice these are recovered directly. -/
+/-- The AST node ids marked as definition sites in a `DefSet`. With the
+`Finset`-of-AST-ids lattice these are just the elements of the set. -/
 def defSites (prog : Program) (d : DefSet prog) : List prog.NodeId :=
-  (List.finRange prog.size).filter (fun i => d i)
+  (List.finRange prog.size).filter (fun i => decide (i ∈ d))
 
 /-- Is the candidate assignment loop-invariant: do all reaching definitions of
 its RHS variables lie outside the loop body? Reaching sets are now keyed by AST