[PowerPC] Canonicalize shuffles on big endian targets as well

Extend shuffle canonicalization and conversion of shuffles fed by vectorized
scalars to big endian subtargets. For big endian subtargets, loads and direct
moves of scalars into vector registers put the data in the correct element for
SCALAR_TO_VECTOR if the data type is 8 bytes wide. However, if the data type is
narrower, the value still ends up in the wrong place - althouth a different
wrong place than on little endian targets.

This patch extends the combine that keeps values where they are if they feed a
shuffle to big endian targets.

Differential revision: https://reviews.llvm.org/D100478