init_reg_autoinc bug and how to describe regs that can't autoincdec

[Richard Kenner]

> I don't understand what you mean; there's a misunderstanding
> here.  To be very brief, I can't have (post_inc (reg:SI 15)) and
> a gdb session tells me this code is responsible for making sure
> that's a valid allocation.

You can't have that post_inc in the post-reload code, certainly,
but if you have a post_inc of a pseudo that's assigned reg 15 and that
register isn't valid for a post_inc, the normal reload mechanism should
generate the in and out reloads to put reg 15 in a reload register of the
appropriate class.