[PowerPC 3/4] Little-endian adjustments for VSX vector shuffle

When performing instruction selection for ISD::VECTOR_SHUFFLE, there
is special code for handling v2f64 and v2i64 using VSX instructions.
This code must be adjusted for little-endian.  Because the two inputs
are treated as a double-wide register, we must swap their order for
little endian.  To get the appropriate mask elements to use with the
big-endian biased XXPERMDI instruction, we must reverse their order
and invert the bits.

A new test is added to test the 16 possible values of the shuffle
mask.  It is initially disabled for reasons specified in the test.  It
is re-enabled by patch 4/4.