Re-reland: Remove register index/code indirection

src/compiler/code-generator-impl.h

Index: src/compiler/code-generator-impl.h
diff --git a/src/compiler/code-generator-impl.h b/src/compiler/code-generator-impl.h
index 83cbd22604a9fb921f6d58746f1d32128c4a0334..2e95bec0cc17188fbfc8d1997bbba90771d7fc3a 100644
--- a/src/compiler/code-generator-impl.h
+++ b/src/compiler/code-generator-impl.h
@@ -96,12 +96,11 @@ class InstructionOperandConverter {
   }
 
   Register ToRegister(InstructionOperand* op) {
-    return Register::FromAllocationIndex(RegisterOperand::cast(op)->index());
+    return RegisterOperand::cast(op)->GetRegister();
   }
 
   DoubleRegister ToDoubleRegister(InstructionOperand* op) {
-    return DoubleRegister::FromAllocationIndex(
-        DoubleRegisterOperand::cast(op)->index());
+    return DoubleRegisterOperand::cast(op)->GetDoubleRegister();
   }
 
   Constant ToConstant(InstructionOperand* op) {