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

Some implementations of std::lower_bound require weak-strict ordering.

The comparison operator must be assymetric, which doesn't hold for less_equals.

Review URL: https://codereview.chromium.org/1232613002

Cr-Commit-Position: refs/heads/master@{#29633}

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());