[PowerPC] Swap arguments and adjust shift count for vsldoi on little endian

Commits r213915 and r214718 fix recognition of shuffle masks for vmrg*
and vpku*um instructions for a little-endian target, by swapping the
input arguments.  The vsldoi instruction requires similar treatment,
and also needs its shift count adjusted for little endian.

Reviewed by Ulrich Weigand.

This is a bug fix candidate for release 3.5 (and hopefully the last of
those for PowerPC).