Const64 gets lowered to MOVDconst.
Change rules using interior Const64 to use MOVDconst instead,
to be less dependent on rule application order.
As a result of doing this, some of the rules end up being
exact duplicates; remove those.
We had those exact duplicates because of the order dependency;
ppc64 had no way to optimize away shifts by a constant
if the initial lowering didn't catch it.
Add those optimizations as well.
The outcome is the same, but this makes the overall rules more robust.