rearranging REG_ALLOC_ORDER in gcc/config/rs6000/rs6000.h

[David Edelsohn]

>>>>> GP writes:

GP> We have found that re-arranging the REG_ALLOC_ORDER in rs6000.h so that all 
GP> the FP registers come after the integer registers greatly reduces the 
GP> tendency of the compiler to generate code that moves 8-byte quantites through 
GP> the FP registers. This is good for code that may run both on targets both 
GP> with and without an FPU, minimizing the performance hit of using an fpu-
GP> emulator.

	Thanks for contributing another patch to avoid most uses of FPRs
in moves of integer quantities.  Previous discussions about this issue
seem to come to the conclusion that a "-mno-fp-moves" flag must ensure
that no FPRs are used in integer code, not merely reduce the tendency of
the compiler to use FPRs.  Safely implementing the former is non-trivial
and probably cannot be done in the target-specific portion of the
compiler. 

Thanks, David