new auto-inc-dec pass

[Joern Rennecke]

In http://gcc.gnu.org/ml/gcc/2007-03/msg00128.html, you wrote:
> One case is about multiple increments, the tree optimizer merges them and
> increments the register only once, so if one only looks at the size of the
> pointer value one misses them, e.g. something like this:
>
> 	(set (mem (reg)) (x))
> 	(set (mem (plus (reg) (const_int 4))) (x))
> 	(set (reg) (plus (reg) (const_int 8))

The patches attached to PR20211 handle this case.