Move some sub-register index calculations to CodeGenRegisters.cpp

Move some sub-register index calculations to CodeGenRegisters.cpp

Create a new CodeGenRegBank class that will eventually hold all the code
that computes the register structure from Records.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132849 91177308-0d34-0410-b5e6-96231b3b80d8