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

src/compiler/instruction.cc

Index: src/compiler/instruction.cc
diff --git a/src/compiler/instruction.cc b/src/compiler/instruction.cc
index 83b45b39ddec69baaac0d80e37c58654c5b49933..9aebb9a17a5389d2f7c0fbb3f45715c2c137a8cf 100644
--- a/src/compiler/instruction.cc
+++ b/src/compiler/instruction.cc
@@ -566,9 +566,12 @@ 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());