[PPC64LE] Correct vperm -> shuffle transform for little endian

As discussed in cfe commit r210279, the correct little-endian
semantics for the vec_perm Altivec interfaces are implemented by
reversing the order of the input vectors and complementing the permute
control vector.  This converts the desired permute from little endian
element order into the big endian element order that the underlying
PowerPC vperm instruction uses.  This is represented with a
ppc_altivec_vperm intrinsic function.

The instruction combining pass contains code to convert a
ppc_altivec_vperm intrinsic into a vector shuffle operation when the
intrinsic has a permute control vector (mask) that is a constant.
However, the vector shuffle operation assumes that vector elements are
in natural order for their endianness, so for little endian code we
will get the wrong result with the existing transformation.

This patch reverses the semantic change to vec_perm that was performed
in altivec.h by once again swapping the input operands and
complementing the permute control vector, returning the element
ordering to little endian.

The correctness of this code is tested by the new perm.c test added in
a previous patch, and by other tests in the test suite that fail
without this patch.