[PowerPC] Disable permuted SCALAR_TO_VECTOR on LE without direct moves

There are some patterns involving the permuted scalar to vector node
for which we don't have patterns without direct moves on little endian
subtargets. This causes selection errors. While we can of course add
the missing patterns, any additional effort to make this work is not
useful since there is no support for any CPU that can run in
little endian mode and does not support direct moves.