[Hexagon] Better determination of register classes in bit tracker

Add two callbacks to MachineEvaluator, so that specific implementations
can specify more details about register classes:
- composeWithSubRegIndex(RC,Idx), to provide the register class for a
  register from RC used in conjunction with a subregister index Idx.
- getPhysRegBitWidth(Reg), to provide the size in bits of the given
  physical register.

llvm-svn: 314136