Revert of Remove register index/code indirection

src/arm64/lithium-arm64.cc

Index: src/arm64/lithium-arm64.cc
diff --git a/src/arm64/lithium-arm64.cc b/src/arm64/lithium-arm64.cc
index 5cc5b9d592eedc19a49ed4a2315a42f862d047e0..e623718a1a323b59e9f672d335e08d22533f6ab3 100644
--- a/src/arm64/lithium-arm64.cc
+++ b/src/arm64/lithium-arm64.cc
@@ -375,13 +375,14 @@
 
 
 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));
 }