Reland "[turbofan] re-wire Unreachable to the graph end at EffectPhi's"

This is a reland of 2c0b1f6e

This fixes two bugs:
- Unreachable might have value uses even after being connected
to Throw, so the solution is to just not replace them with the Dead node
anymore.
- We didn't trigger initial visitation of the new Throw node.
  Re-visiting the changed End node takes care of this.


Original change's description:
> [turbofan] re-wire Unreachable to the graph end at EffectPhi's
>
> This avoids the EffectControlLinearizer stumbling upon unreachable
> code.
>
> Bug: chromium:958718
> Change-Id: I135c17813741e48e878a4624370eee1e06081031
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1605737
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61434}

Bug: chromium:958718 chromium:962475 chromium:962474
Change-Id: I388a59912e6260a221cccc76102e0c4b00bff93e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1609791Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61482}

src/compiler/dead-code-elimination.cc

@@ -61,7 +61,7 @@ Reduction DeadCodeElimination::Reduce(Node* node) {
     case IrOpcode::kPhi:
       return ReducePhi(node);
     case IrOpcode::kEffectPhi:
-      return PropagateDeadControl(node);
+      return ReduceEffectPhi(node);
     case IrOpcode::kDeoptimize:
     case IrOpcode::kReturn:
     case IrOpcode::kTerminate:
@@ -109,7 +109,6 @@ Reduction DeadCodeElimination::ReduceEnd(Node* node) {
   return NoChange();
 }
 
-
 Reduction DeadCodeElimination::ReduceLoopOrMerge(Node* node) {
   DCHECK(IrOpcode::IsMergeOpcode(node->opcode()));
   Node::Inputs inputs = node->inputs();
@@ -233,6 +232,34 @@ Reduction DeadCodeElimination::ReducePhi(Node* node) {
   return NoChange();
 }
 
+Reduction DeadCodeElimination::ReduceEffectPhi(Node* node) {
+  DCHECK_EQ(IrOpcode::kEffectPhi, node->opcode());
+  Reduction reduction = PropagateDeadControl(node);
+  if (reduction.Changed()) return reduction;
+
+  Node* merge = NodeProperties::GetControlInput(node);
+  DCHECK(merge->opcode() == IrOpcode::kMerge ||
+         merge->opcode() == IrOpcode::kLoop);
+  int input_count = node->op()->EffectInputCount();
+  for (int i = 0; i < input_count; ++i) {
+    Node* effect = NodeProperties::GetEffectInput(node, i);
+    if (effect->opcode() == IrOpcode::kUnreachable) {
+      // If Unreachable hits an effect phi, we can re-connect the effect chain
+      // to the graph end and delete the corresponding inputs from the merge and
+      // phi nodes.
+      Node* control = NodeProperties::GetControlInput(merge, i);
+      Node* throw_node = graph_->NewNode(common_->Throw(), effect, control);
+      NodeProperties::MergeControlToEnd(graph_, common_, throw_node);
+      NodeProperties::ReplaceEffectInput(node, dead_, i);
+      NodeProperties::ReplaceControlInput(merge, dead_, i);
+      Revisit(merge);
+      Revisit(graph_->end());
+      reduction = Changed(node);
+    }
+  }
+  return reduction;
+}
+
 Reduction DeadCodeElimination::ReducePureNode(Node* node) {
   DCHECK_EQ(0, node->op()->EffectInputCount());
   if (node->opcode() == IrOpcode::kDeadValue) return NoChange();

src/compiler/dead-code-elimination.h

@@ -53,6 +53,7 @@ class V8_EXPORT_PRIVATE DeadCodeElimination final
   Reduction ReduceLoopExit(Node* node);
   Reduction ReduceNode(Node* node);
   Reduction ReducePhi(Node* node);
+  Reduction ReduceEffectPhi(Node* node);
   Reduction ReducePureNode(Node* node);
   Reduction ReduceUnreachableOrIfException(Node* node);
   Reduction ReduceEffectNode(Node* node);