Move spill size and alignment info from MC to TargetRegisterInfo

This is another step towards implementing register classes with
parametrized register/spill sizes and value types.

This is an updated version of r298652. The difference is that MCRegister-
Class still contains register size, available as getPhysRegSize(). The
old function getSize was retained as a temporary measure to avoid build
breakage for out-of-tree targets.

llvm-svn: 298739