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

Unfortunately this is another regression from my canonicalization patch
(1fed1316). The patch contained two implicit assumptions:
1. That we would have a permuted load only if we are loading a partial vector
2. 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.

(cherry picked from commit 7d076e19)