[PowerPC] Fix computation of offset for load-and-splat for permuted loads

Unfortunately this is another regression from my canonicalization patch

(1fed131660b2). The patch contained two implicit assumptions:

- That we would have a permuted load only if we are loading a partial vector
- That a partial vector load would necessarily be as wide as the splat

However, assumption 2 is not correct since it is possible to do a wider

load and only splat a half of it. This patch corrects this assumption by

simply checking if the load is permuted and adjusting the offset if it is.