RegisterPressure API. Add support for physical register units.

At build-time register pressure was always computed in terms of
register units. But the compile-time API was expressed in terms of
register classes because it was intended for virtual registers (and
physical register units weren't yet used anywhere in codegen).

Now that the codegen uses physreg units consistently, prepare for
tracking register pressure also in terms of live units, not live
registers.

llvm-svn: 169360