Revert of [turbofan] simplify gap ordering

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index 48dbdf20e70fdd6bf9f8b83991edf6417ef8ca1c..9e2004fa88bc5cad6aad1013773092b2f02d565e 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -25,7 +25,8 @@
 
 // A couple of reserved opcodes are used for internal use.
 const InstructionCode kGapInstruction = -1;
-const InstructionCode kSourcePositionInstruction = -2;
+const InstructionCode kBlockStartInstruction = -2;
+const InstructionCode kSourcePositionInstruction = -3;
 
 #define INSTRUCTION_OPERAND_LIST(V)                                  \
   V(Constant, CONSTANT, 0)                                           \
@@ -491,7 +492,10 @@
   bool NeedsPointerMap() const { return IsCall(); }
   bool HasPointerMap() const { return pointer_map_ != NULL; }
 
-  bool IsGapMoves() const { return opcode() == kGapInstruction; }
+  bool IsGapMoves() const {
+    return opcode() == kGapInstruction || opcode() == kBlockStartInstruction;
+  }
+  bool IsBlockStart() const { return opcode() == kBlockStartInstruction; }
   bool IsSourcePosition() const {
     return opcode() == kSourcePositionInstruction;
   }
@@ -637,6 +641,30 @@
 };
 
 
+// This special kind of gap move instruction represents the beginning of a
+// block of code.
+class BlockStartInstruction FINAL : public GapInstruction {
+ public:
+  static BlockStartInstruction* New(Zone* zone) {
+    void* buffer = zone->New(sizeof(BlockStartInstruction));
+    return new (buffer) BlockStartInstruction();
+  }
+
+  static BlockStartInstruction* cast(Instruction* instr) {
+    DCHECK(instr->IsBlockStart());
+    return static_cast<BlockStartInstruction*>(instr);
+  }
+
+  static const BlockStartInstruction* cast(const Instruction* instr) {
+    DCHECK(instr->IsBlockStart());
+    return static_cast<const BlockStartInstruction*>(instr);
+  }
+
+ private:
+  BlockStartInstruction() : GapInstruction(kBlockStartInstruction) {}
+};
+
+
 class SourcePositionInstruction FINAL : public Instruction {
  public:
   static SourcePositionInstruction* New(Zone* zone, SourcePosition position) {
@@ -952,7 +980,7 @@
 
   void AddGapMove(int index, InstructionOperand* from, InstructionOperand* to);
 
-  GapInstruction* GetBlockStart(BasicBlock::RpoNumber rpo) const;
+  BlockStartInstruction* GetBlockStart(BasicBlock::RpoNumber rpo) const;
 
   typedef InstructionDeque::const_iterator const_iterator;
   const_iterator begin() const { return instructions_.begin(); }