recommit:[PowerPC] Eliminate loads/swap feeding swap/store for vector type by...

recommit:[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.

Differential Revision: https://reviews.llvm.org/D65063

llvm-svn: 367516