[PowerPC] extend PPCPreIncPrep Pass for ds/dq form

Now, PPCPreIncPrep pass changes a loop to update form and update all load/store
with same base accordingly. We can do more for load/store with same base, for
example, convert load/store with same base to ds/dq form.

Reviewed by: jsji

Differential Revision: https://reviews.llvm.org/D67088