RFA: Some fixes in dfa ppc8450 description

[David Edelsohn]

        * config/rs6000/rs6000.c
        (TARGET_SCHED_FIRST_CYCLE_MULTIPASS_DFA_LOOKAHEAD): Redefine the
        macros.
        (rs6000_issue_rate): Add case for 8540.
        (rs6000_use_sched_lookahead): New function.

        * config/rs6000/8540.md: Rename SIU units into SU ones and MIU
        units into MU ones.
        (ppc8540_branch, ppc8540_cr_logical): Add one cycle in the
        reservation before retirement.
        (ppc8540_multiply, ppc8540_load, ppc8540_store,
        ppc8540_simple_float, ppc8540_vector_load, ppc8540_vector_store):
        Remove additional cycle in the reservation before retirement.
        (ppc8540_mfcr, ppc8540_mtcrf, ppc8540_mtjmpr): Add missed
        reservation of ppc8540_issue.

Okay.

	Does enabling lookahead actually improve performance in reality,
not just theory?  It turned out to be a lose for Power4.

Thanks, David