default_secondary_reload: class vs scratch_class

[DJ Delorie]

> What reason is there to have scratch_class be something else?

SECONDARY_RELOAD_CLASS has the option of limiting the reload class.
The mn10300 has a generic SImode reload_in that allows GENERAL_REGS,
but SECONDARY_RELOAD_CLASS specifies a smaller class based on the
registers that need reloading.

The default hook for secondary_reload uses SECONDARY_RELOAD_CLASS but
assumes it's going to return the same class as the pattern (in which
case, why bother defining it?).  Since we're only allowed one reload
pattern per mode, this seems like an artificial limitation.