Change to readonly conflict handling

[Richard Kenner]

    This code is actually not valid C; neither of the two assignments are
    valid, because they change p->a and q->a which are 'const'.  

Yes, I know, but I don't think there's any way to get it from valid C.  This
was meant to be as close I could get the actual case, which was from Ada.

    Actually, what I get is _both_ words of *p and *q being stored with
    /u.  On powerpc (so the fields are SImode):

Right, I get that too.  I even think I see why, though it is a bug.


(insn 16 15 19 (set (mem/s/u:DI (reg/v:SI 82) 0)
        (reg:DI 86)) -1 (nil)
    (nil))

(insn 19 16 20 (set (reg:DI 87)
        (mem/s:DI (reg/v:SI 82) 0)) -1 (nil)
    (nil))

But *this* is key: it's being stored *with* the "/u", but loaded without.
That will cause these two loads to be marked non-conflicting, which may
have them scheduled in the wrong order (how I ran into the bug).