[turbofan] Introduce separate SelectLowering reducer.

src/compiler/select-lowering.cc

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/compiler/select-lowering.h"
+
+#include "src/compiler/common-operator.h"
+#include "src/compiler/generic-node-inl.h"
+#include "src/compiler/graph.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+SelectLowering::SelectLowering(Graph* graph, CommonOperatorBuilder* common)
+    : common_(common),
+      graph_(graph),
+      merges_(Merges::key_compare(), Merges::allocator_type(graph->zone())) {}
+
+
+SelectLowering::~SelectLowering() {}
+
+
+Reduction SelectLowering::Reduce(Node* node) {
+  if (node->opcode() != IrOpcode::kSelect) return NoChange();
+  SelectParameters const p = SelectParametersOf(node->op());
+
+  Node* const cond = node->InputAt(0);
+  Node* const control = graph()->start();
+
+  // Check if we already have a diamond for this condition.
+  auto i = merges_.find(cond);
+  if (i == merges_.end()) {
+    // Create a new diamond for this condition and remember its merge node.
+    Node* branch = graph()->NewNode(common()->Branch(p.hint()), cond, control);
+    Node* if_true = graph()->NewNode(common()->IfTrue(), branch);
+    Node* if_false = graph()->NewNode(common()->IfFalse(), branch);
+    Node* merge = graph()->NewNode(common()->Merge(2), if_true, if_false);
+    i = merges_.insert(std::make_pair(cond, merge)).first;
+  }
+
+  DCHECK_EQ(cond, i->first);
+
+  // Create a Phi hanging off the previously determined merge.
+  node->set_op(common()->Phi(p.type(), 2));
+  node->ReplaceInput(0, node->InputAt(1));
+  node->ReplaceInput(1, node->InputAt(2));
+  node->ReplaceInput(2, i->second);
+  return Changed(node);
+}
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8