Regression due to tree loop opt / expand weirdness

[Richard Kenner]

    First of all, the above change is provably a no-op on ia64 since
    the valid_pointer_mode implementation on ia64 is the default one,
    which is exactly (mode != Pmode && mode != ptr_mode).

Please read what I wrote.  On some ia64 systems (e.g., VMS), Pmode and
ptr_mode are both DImode but you need to support 32-bit pointers.  On
those ia64 systems, target.valid_pointer_mode is overridden to allow both
SImode and DImode.