Revert of Remove register index/code indirection

src/ia32/lithium-ia32.cc

Index: src/ia32/lithium-ia32.cc
diff --git a/src/ia32/lithium-ia32.cc b/src/ia32/lithium-ia32.cc
index 262e16bcbd8c5230ce5f8bcc7d5c3af3df379fde..884067b776e417e086b948c93d52d9d362d78083 100644
--- a/src/ia32/lithium-ia32.cc
+++ b/src/ia32/lithium-ia32.cc
@@ -482,13 +482,14 @@
 
 
 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));
 }