[PowerPC] Eliminate loads/swap feeding swap/store for vector type by using big-endian...

[PowerPC] Eliminate loads/swap feeding swap/store for vector type by using big-endian load/store

In PowerPC, there is instruction to load vector in big endian element order when it's in little endian target.
So we can combine vector load + reverse into big endian load to eliminate the swap instruction.
Also combine vector reverse + store into big endian store.

llvm-svn: 367382