CodeGen: Introduce a class for registers

Avoids using a plain unsigned for registers throughoug codegen.
Doesn't attempt to change every register use, just something a little
more than the set needed to build after changing the return type of
MachineOperand::getReg().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364191 91177308-0d34-0410-b5e6-96231b3b80d8