Add a level of indirection to the register accessors used in

Add a level of indirection to the register accessors used in
instruction execute methods.  Register i now means the instruction's
i'th src (or dest) operand, not architectural register i.  Current
models that use the architectural reg index can look that up easily
in the instruction object.  Future models that do register renaming
should find this much simpler to deal with.

arch/isa_parser.py:
    Generate register accessors with an extra level of indirection.
cpu/simple_cpu/simple_cpu.hh:
    Modify register accessors to use an extra level of indirection.

--HG--
extra : convert_revision : f4c7d6bfa92fb2ea6251f31ee368809c3643f08f