rs6000: PREFERRED_RELOAD_CLASS

[David Edelsohn]

>>>>> Geoff Keating writes:

Geoff> Anyway, we still don't want to be performing memory copies using FPRs.
Geoff> The latency makes it faster to do the copy using GPRs.

	This is not always the case and one may want the bandwidth over
the latency.  I do not think that making this change so cavalierly is
wise.

	Also, I believe that some parts of the PowerPC port make decisions
based on moving 8 byte small structures in FPRs.  I am not sure how
HARD_REGNO_OK versus PREFFERED_RELOAD_CLASS interact to allow or prevent
that after your change.  I am very concerned about this change and the
possibility that it will have unintended negative consequences.

David