Revert of Remove register index/code indirection

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 722ab01be2b5396b98de40a7967c346cce5dd83a..9df3a7dabf328745bf4731be4f0dd46ed97d8772 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -470,13 +470,14 @@
 
 
 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(XMMRegister reg) {
-  return new (zone())
-      LUnallocated(LUnallocated::FIXED_DOUBLE_REGISTER, reg.code());
+  return new(zone()) LUnallocated(LUnallocated::FIXED_DOUBLE_REGISTER,
+                                  XMMRegister::ToAllocationIndex(reg));
 }