[turbofan] Implement jump threading after register allocation.

src/compiler/instruction.cc

Index: src/compiler/instruction.cc
diff --git a/src/compiler/instruction.cc b/src/compiler/instruction.cc
index aa0739e4068fd4c8670ac58e0c342849599908f6..3afc970e992128f86ba5fd028fd0ae017bcc8a22 100644
--- a/src/compiler/instruction.cc
+++ b/src/compiler/instruction.cc
@@ -120,6 +120,16 @@ bool ParallelMove::IsRedundant() const {
 }
 
 
+bool GapInstruction::IsRedundant() const {
+  for (int i = GapInstruction::FIRST_INNER_POSITION;
+       i <= GapInstruction::LAST_INNER_POSITION; i++) {
+    if (parallel_moves_[i] != NULL && !parallel_moves_[i]->IsRedundant())
+      return false;
+  }
+  return true;
+}
+
+
 std::ostream& operator<<(std::ostream& os,
                          const PrintableParallelMove& printable) {
   const ParallelMove& pm = *printable.parallel_move_;