[turbofan] Remove some clever-but-wrong bits from select lowering.

src/compiler/select-lowering.cc

Index: src/compiler/select-lowering.cc
diff --git a/src/compiler/select-lowering.cc b/src/compiler/select-lowering.cc
index e5d86c28f0c09bf378086f3892aa3fc5be1281bb..b1a230962fd511aca828838a198cbf60df21ff97 100644
--- a/src/compiler/select-lowering.cc
+++ b/src/compiler/select-lowering.cc
@@ -15,10 +15,7 @@ namespace internal {
 namespace compiler {
 
 SelectLowering::SelectLowering(Graph* graph, CommonOperatorBuilder* common)
-    : common_(common),
-      graph_(graph),
-      merges_(Merges::key_compare(), Merges::allocator_type(graph->zone())) {}
-
+    : common_(common), graph_(graph) {}
 
 SelectLowering::~SelectLowering() {}
 
@@ -30,58 +27,16 @@ Reduction SelectLowering::Reduce(Node* node) {
   Node* cond = node->InputAt(0);
   Node* vthen = node->InputAt(1);
   Node* velse = node->InputAt(2);
-  Node* merge = nullptr;
-
-  // Check if we already have a diamond for this condition.
-  auto range = merges_.equal_range(cond);
-  for (auto i = range.first;; ++i) {
-    if (i == range.second) {
-      // Create a new diamond for this condition and remember its merge node.
-      Diamond d(graph(), common(), cond, p.hint());
-      merges_.insert(std::make_pair(cond, d.merge));
-      merge = d.merge;
-      break;
-    }
-
-    // If the diamond is reachable from the Select, merging them would result in
-    // an unschedulable graph, so we cannot reuse the diamond in that case.
-    merge = i->second;
-    if (!ReachableFrom(merge, node)) {
-      break;
-    }
-  }
 
-  // Create a Phi hanging off the previously determined merge.
+  // Create a diamond and a phi.
+  Diamond d(graph(), common(), cond, p.hint());
   node->ReplaceInput(0, vthen);
   node->ReplaceInput(1, velse);
-  node->ReplaceInput(2, merge);
+  node->ReplaceInput(2, d.merge);
   NodeProperties::ChangeOp(node, common()->Phi(p.representation(), 2));
   return Changed(node);
 }
 
-
-bool SelectLowering::ReachableFrom(Node* const sink, Node* const source) {
-  // TODO(turbofan): This is probably horribly expensive, and it should be moved
-  // into node.h or somewhere else?!
-  Zone zone(graph()->zone()->allocator());
-  std::queue<Node*, NodeDeque> queue((NodeDeque(&zone)));
-  BoolVector visited(graph()->NodeCount(), false, &zone);
-  queue.push(source);
-  visited[source->id()] = true;
-  while (!queue.empty()) {
-    Node* current = queue.front();
-    if (current == sink) return true;
-    queue.pop();
-    for (auto input : current->inputs()) {
-      if (!visited[input->id()]) {
-        queue.push(input);
-        visited[input->id()] = true;
-      }
-    }
-  }
-  return false;
-}
-
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8