[Turbofan] Reduce register allocation work when we can.

src/compiler/instruction.cc

Index: src/compiler/instruction.cc
diff --git a/src/compiler/instruction.cc b/src/compiler/instruction.cc
index 174ee300fee63348fbf0791801528d628da2b2a2..142da9bf76dd65fc1edf337cc30d15bd8f142aae 100644
--- a/src/compiler/instruction.cc
+++ b/src/compiler/instruction.cc
@@ -813,6 +813,7 @@ InstructionSequence::InstructionSequence(Isolate* isolate,
       next_virtual_register_(0),
       reference_maps_(zone()),
       representations_(zone()),
+      representation_mask_(0),
       deoptimization_entries_(zone()),
       current_block_(nullptr) {}
 
@@ -918,6 +919,7 @@ void InstructionSequence::MarkAsRepresentation(MachineRepresentation rep,
   DCHECK_IMPLIES(representations_[virtual_register] != rep,
                  representations_[virtual_register] == DefaultRepresentation());
   representations_[virtual_register] = rep;
+  representation_mask_ |= 1 << static_cast<int>(rep);
 }
 
 int InstructionSequence::AddDeoptimizationEntry(