[turbofan] Teach ad-hoc load elimination about atomic regions.

The LoadElimination in TurboFan can look into the atomic regions and
elimination subsequent loads based on stores/allocations in that atomic
regions.

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1472703005

Cr-Commit-Position: refs/heads/master@{#32179}

src/compiler/load-elimination.cc

@@ -28,7 +28,7 @@ Reduction LoadElimination::Reduce(Node* node) {
 Reduction LoadElimination::ReduceLoadField(Node* node) {
   DCHECK_EQ(IrOpcode::kLoadField, node->opcode());
   FieldAccess const access = FieldAccessOf(node->op());
-  Node* const object = NodeProperties::GetValueInput(node, 0);
+  Node* object = NodeProperties::GetValueInput(node, 0);
   for (Node* effect = NodeProperties::GetEffectInput(node);;
        effect = NodeProperties::GetEffectInput(effect)) {
     switch (effect->opcode()) {
@@ -53,11 +53,24 @@ Reduction LoadElimination::ReduceLoadField(Node* node) {
         }
         break;
       }
+      case IrOpcode::kBeginRegion:
       case IrOpcode::kStoreBuffer:
       case IrOpcode::kStoreElement: {
         // These can never interfere with field loads.
         break;
       }
+      case IrOpcode::kFinishRegion: {
+        // "Look through" FinishRegion nodes to make LoadElimination capable
+        // of looking into atomic regions.
+        if (object == effect) object = NodeProperties::GetValueInput(effect, 0);
+        break;
+      }
+      case IrOpcode::kAllocate: {
+        // Allocations don't interfere with field loads. In case we see the
+        // actual allocation for the {object} we can abort.
+        if (object == effect) return NoChange();
+        break;
+      }
       default: {
         if (!effect->op()->HasProperty(Operator::kNoWrite) ||
             effect->op()->EffectInputCount() != 1) {