Autoincrement addressing modes

[Joern Rennecke]

> Does this asymmetry only apply to these update forms of addressing
> modes?
> 
> Any ideas how to describe this to GCC?  I suppose if there was a flag

One way would be to have

HAVE_PRE_DECREMENT_READ and HAVE_PRE_DECREMENT_WRITE to control the
two possible uses of PRE_DEC;
when HAVE_PRE_DECREMENT is defined, it can mean both of the above,
to maintain backward compatibility and make it easy for ports that
don't need to distinguish them.
Similarily for POST_DEC, PRE_INC, POST_INC.

> to differentiate memory reads from writes, then
> GO_IF_LEGITIMATE_ADDRESS could reject the invalid forms.

That would be goof too, but it doesn't help the code that doesn't
actually try to use the auto-increment, e.g. like move_by_pieces
in expr.c