[turbofan] add register assignment verifier

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index b6fcb3c074aaa8a441d38e7f5c7746c0f8a34857..8a6006a424dea35fb8e3a55929f3b9484ec3bf06 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -332,6 +332,11 @@ class SubKindOperand FINAL : public InstructionOperand {
     return reinterpret_cast<SubKindOperand*>(op);
   }
 
+  static const SubKindOperand* cast(const InstructionOperand* op) {
+    DCHECK(op->kind() == kOperandKind);
+    return reinterpret_cast<const SubKindOperand*>(op);
+  }
+
   static void SetUpCache();
   static void TearDownCache();
 
@@ -581,7 +586,7 @@ class GapInstruction : public Instruction {
     return parallel_moves_[pos];
   }
 
-  ParallelMove* GetParallelMove(InnerPosition pos) {
+  ParallelMove* GetParallelMove(InnerPosition pos) const {
     return parallel_moves_[pos];
   }
 
@@ -917,6 +922,7 @@ class InstructionSequence FINAL {
   typedef InstructionDeque::const_iterator const_iterator;
   const_iterator begin() const { return instructions_.begin(); }
   const_iterator end() const { return instructions_.end(); }
+  const InstructionDeque& instructions() const { return instructions_; }
 
   GapInstruction* GapAt(int index) const {
     return GapInstruction::cast(InstructionAt(index));