[PPC64LE] Fix lowering of BUILD_VECTOR and SHUFFLE_VECTOR for little endian

This patch fixes a couple of lowering issues for little endian
PowerPC.  The code for lowering BUILD_VECTOR contains a number of
optimizations that are only valid for big endian.  For now, we disable
those optimizations for correctness.  In the future, we will add
analogous optimizations that are correct for little endian.

When lowering a SHUFFLE_VECTOR to a VPERM operation, we again need to
make the now-familiar transformation of swapping the input operands
and complementing the permute control vector.  Correctness of this
transformation is tested by the accompanying test case.