RAGreedyStats: Ignore identity COPYs; count COPYs from/to physregs

Improve copy statistics:

- Count copies from or to physical registers: They are used to model function parameters and calling conventions and the register allocator optimizes for them.
- Check physical registers assigned to virtual registers and stop counting "identity" `COPY`s where source and destination is the same physical registers; they will be removed in the `virtregmap` pass anyway.

Differential Revision: https://reviews.llvm.org/D131932