Bump virtual register width in LUnallocated.

src/lithium.h

Index: src/lithium.h
diff --git a/src/lithium.h b/src/lithium.h
index 25229991a6c2e338b9b7438d80be85bd0b6315d4..388f5658db67c45b2b87f7841c6d0d124c2252c6 100644
--- a/src/lithium.h
+++ b/src/lithium.h
@@ -182,8 +182,7 @@ class LUnallocated: public LOperand {
 
   // BitFields for all unallocated operands.
   class BasicPolicyField     : public BitField<BasicPolicy,     3,  1> {};
-  // TODO(mstarzinger): Bump this from 15 bit to 18 bit in a follow-up CL.
-  class VirtualRegisterField : public BitField<unsigned,        4, 15> {};
+  class VirtualRegisterField : public BitField<unsigned,        4, 18> {};
 
   // BitFields specific to BasicPolicy::FIXED_SLOT.
   class FixedSlotIndexField  : public BitField<int,            22, 10> {};