[turbofan] Fix undefined behavior in InstructionSequence::GetInstructionBlock.

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/graph.h"
 #include "src/compiler/instruction.h"
 #include "src/compiler/schedule.h"
 
 namespace v8 {
@@ skipping 548 matching lines @@
     reference_maps_.push_back(reference_map);
   }
   return index;
 }
 
 
 InstructionBlock* InstructionSequence::GetInstructionBlock(
     int instruction_index) const {
   DCHECK(instruction_blocks_->size() == block_starts_.size());
   auto begin = block_starts_.begin();
-  auto end = std::lower_bound(begin, block_starts_.end(), instruction_index,
-                              std::less_equal<int>());
-  size_t index = std::distance(begin, end) - 1;
+  auto end = std::lower_bound(begin, block_starts_.end(), instruction_index);
+  // Post condition of std::lower_bound:
+  DCHECK(end == block_starts_.end() || *end >= instruction_index);
+  if (end == block_starts_.end() || *end > instruction_index) --end;
+  DCHECK(*end <= instruction_index);
+  size_t index = std::distance(begin, end);
   auto block = instruction_blocks_->at(index);
   DCHECK(block->code_start() <= instruction_index &&
          instruction_index < block->code_end());
   return block;
 }
 
 
 static MachineType FilterRepresentation(MachineType rep) {
   DCHECK_EQ(rep, RepresentationOf(rep));
   switch (rep) {
@@ skipping 223 matching lines @@
       os << " B" << succ.ToInt();
     }
     os << "\n";
   }
   return os;
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8