Revert of Remove register index/code indirection

src/mips/lithium-mips.cc

Index: src/mips/lithium-mips.cc
diff --git a/src/mips/lithium-mips.cc b/src/mips/lithium-mips.cc
index cafa4cfdbc22858b42e26a15b36f161aef7722dc..42ecab4b8e3c205724760b97bf5bfbd269c0d7a8 100644
--- a/src/mips/lithium-mips.cc
+++ b/src/mips/lithium-mips.cc
@@ -451,13 +451,14 @@
 
 
 LUnallocated* LChunkBuilder::ToUnallocated(Register reg) {
-  return new (zone()) LUnallocated(LUnallocated::FIXED_REGISTER, reg.code());
+  return new(zone()) LUnallocated(LUnallocated::FIXED_REGISTER,
+                                  Register::ToAllocationIndex(reg));
 }
 
 
 LUnallocated* LChunkBuilder::ToUnallocated(DoubleRegister reg) {
-  return new (zone())
-      LUnallocated(LUnallocated::FIXED_DOUBLE_REGISTER, reg.code());
+  return new(zone()) LUnallocated(LUnallocated::FIXED_DOUBLE_REGISTER,
+                                  DoubleRegister::ToAllocationIndex(reg));
 }