[turbofan] Stand-alone deferred block splitting - full.

src/compiler/instruction.h

Index: src/compiler/instruction.h
diff --git a/src/compiler/instruction.h b/src/compiler/instruction.h
index a87ef7dc9c469f865919aeef34a79fbaacb5e033..304d7499d386a35361bc4a0b1da45f1a574c4603 100644
--- a/src/compiler/instruction.h
+++ b/src/compiler/instruction.h
@@ -1085,6 +1085,9 @@ class InstructionSequence final : public ZoneObject {
   const_iterator begin() const { return instructions_.begin(); }
   const_iterator end() const { return instructions_.end(); }
   const InstructionDeque& instructions() const { return instructions_; }
+  int LastInstructionIndex() const {
+    return static_cast<int>(instructions().size()) - 1;
+  }
 
   Instruction* InstructionAt(int index) const {
     DCHECK(index >= 0);