fix: correctly handle join points with no params in the IR elim_dead_branches pass (#8015)

This PR fixes the IR elim_dead_branches pass to correctly handle join
points with no params, which currently get considered unreachable. I was
not able to find an easy repro of this with the old compiler, but it
occurs when bootstrapping Lean with the new compiler.

src/Lean/Compiler/IR/ElimDeadBranches.lean

@@ -165,6 +165,7 @@ structure InterpContext where
 structure InterpState where
   assignments : Array Assignment
   funVals     : PArray Value -- we take snapshots during fixpoint computations
+  visitedJps  : Array (Std.HashSet JoinPointId)
 
 abbrev M := ReaderT InterpContext (StateM InterpState)
 
@@ -223,11 +224,18 @@ def updateCurrFnSummary (v : Value) : M Unit := do
   let currFnIdx := ctx.currFnIdx
   modify fun s => { s with funVals := s.funVals.modify currFnIdx (fun v' => widening ctx.env v v') }
 
+def markJPVisited (j : JoinPointId) : M Bool := do
+  let currFnIdx := (← read).currFnIdx
+  modifyGet fun s =>
+    ⟨!(s.visitedJps[currFnIdx]!.contains j),
+     { s with visitedJps := s.visitedJps.modify currFnIdx fun a => a.insert j }⟩
+
 /-- Return true if the assignment of at least one parameter has been updated. -/
-def updateJPParamsAssignment (ys : Array Param) (xs : Array Arg) : M Bool := do
+def updateJPParamsAssignment (j : JoinPointId) (ys : Array Param) (xs : Array Arg) : M Bool := do
   let ctx ← read
   let currFnIdx := ctx.currFnIdx
-  ys.size.foldM (init := false) fun i _ r => do
+  let isFirstVisit ← markJPVisited j
+  ys.size.foldM (init := isFirstVisit) fun i _ r => do
     let y := ys[i]
     let x := xs[i]!
     let yVal ← findVarValue y.x
@@ -272,7 +280,7 @@ partial def interpFnBody : FnBody → M Unit
     let ctx ← read
     let ys := (ctx.lctx.getJPParams j).get!
     let b  := (ctx.lctx.getJPBody j).get!
-    let updated ← updateJPParamsAssignment ys xs
+    let updated ← updateJPParamsAssignment j ys xs
     if updated then
       -- We must reset the value of nested join-point parameters since they depend on `ys` values
       resetNestedJPParams b
@@ -283,7 +291,8 @@ partial def interpFnBody : FnBody → M Unit
 
 def inferStep : M Bool := do
   let ctx ← read
-  modify fun s => { s with assignments := ctx.decls.map fun _ => {} }
+  modify fun s => { s with assignments := ctx.decls.map fun _ => {},
+                           visitedJps := ctx.decls.map fun _ => {} }
   ctx.decls.size.foldM (init := false) fun idx _ modified => do
     match ctx.decls[idx] with
     | .fdecl (xs := ys) (body := b) .. => do
@@ -332,8 +341,9 @@ def elimDeadBranches (decls : Array Decl) : CompilerM (Array Decl) := do
   let env := s.env
   let assignments : Array Assignment := decls.map fun _ => {}
   let funVals := mkPArray decls.size Value.bot
+  let visitedJps := decls.map fun _ => {}
   let ctx : InterpContext := { decls := decls, env := env }
-  let s : InterpState := { assignments := assignments, funVals := funVals }
+  let s : InterpState := { assignments, funVals, visitedJps }
   let (_, s) := (inferMain ctx).run s
   let funVals := s.funVals
   let assignments := s.assignments