Reapply r161633-161634 "Partition use lists so defs always come before uses.""

No changes to these patches, MRI needed to be notified when changing
uses into defs and vice versa.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161644 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/MachineOperand.h

@@ -150,7 +150,7 @@ private:
 
     struct {                  // For MO_Register.
       // Register number is in SmallContents.RegNo.
-      MachineOperand **Prev;  // Access list for register.
+      MachineOperand *Prev;   // Access list for register. See MRI.
       MachineOperand *Next;
     } Reg;