[turbofan] Implement jump threading after register allocation.

src/compiler/jump-threading.h

+// 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.
+
+#ifndef V8_COMPILER_JUMP_THREADING_H_
+#define V8_COMPILER_JUMP_THREADING_H_
+
+#include "src/compiler/instruction.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// Forwards jumps to empty basic blocks that end with a second jump to the
+// destination of the second jump, transitively.
+class JumpThreading {
+ public:
+  // Compute the forwarding map of basic blocks to their ultimate destination.
+  // Returns {true} if there is at least one block that is forwarded.
+  static bool ComputeForwarding(Zone* local_zone,
+                                ZoneVector<BasicBlock::RpoNumber>& result,
+                                InstructionSequence* code);
+
+  // Rewrite the instructions to forward jumps and branches.
+  // May also negate some branches.
+  static void ApplyForwarding(ZoneVector<BasicBlock::RpoNumber>& forwarding,
+                              InstructionSequence* code);
+};
+
+}  // namespace compiler
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_COMPILER_JUMP_THREADING_H