Revert of Remove register index/code indirection

src/mips64/lithium-mips64.cc

Index: src/mips64/lithium-mips64.cc
diff --git a/src/mips64/lithium-mips64.cc b/src/mips64/lithium-mips64.cc
index 4595722fb4061775ec1b2a8bec98026ef36495e4..4f2f16152410792da707179b8b60dd61d77875ee 100644
--- a/src/mips64/lithium-mips64.cc
+++ b/src/mips64/lithium-mips64.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));
 }