[turbofan] add gap move verifier

src/compiler/instruction.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/common-operator.h"
 #include "src/compiler/generic-node-inl.h"
 #include "src/compiler/graph.h"
 #include "src/compiler/instruction.h"
 
 namespace v8 {
@@ skipping 417 matching lines @@
       next_virtual_register_(0),
       pointer_maps_(zone()),
       doubles_(std::less<int>(), VirtualRegisterSet::allocator_type(zone())),
       references_(std::less<int>(), VirtualRegisterSet::allocator_type(zone())),
       deoptimization_entries_(zone()) {
   block_starts_.reserve(instruction_blocks_->size());
 }
 
 
 BlockStartInstruction* InstructionSequence::GetBlockStart(
-    BasicBlock::RpoNumber rpo) {
-  InstructionBlock* block = InstructionBlockAt(rpo);
+    BasicBlock::RpoNumber rpo) const {
+  const InstructionBlock* block = InstructionBlockAt(rpo);
   return BlockStartInstruction::cast(InstructionAt(block->code_start()));
 }
 
 
 void InstructionSequence::StartBlock(BasicBlock::RpoNumber rpo) {
   DCHECK(block_starts_.size() == rpo.ToSize());
   InstructionBlock* block = InstructionBlockAt(rpo);
   int code_start = static_cast<int>(instructions_.size());
   block->set_code_start(code_start);
   block_starts_.push_back(code_start);
@@ skipping 217 matching lines @@
       os << " B" << succ_block->id();
     }
     os << "\n";
   }
   return os;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8