Possible bug in cse.c affecting pre/post-modify mem access

[A. Skrobov]

>> That makes me wonder if there is a latent bug though.  Consider pushing
>> args to a pure function.  Could we then try to CSE the memory reference
>> and get it wrong because we haven't accounted for the autoinc?
>
> Can't know for sure but one would hope something would test for
> side_effects_p.

If side_effects_p were checked in all the right places, then our port
(which is more liberal at generating auto-inc insns in early passes)
wouldn't have cse generate incorrect code, right?
Our observation is that it did.