[PowerPC] Enable splat generation for BUILD_VECTOR with little endian

When enabling PPC64LE, I disabled some optimizations of BUILD_VECTOR
nodes for little endian because wrong results were produced.  I've
subsequently investigated and found this is due to a call to
BuildVectorSDNode::isConstantSplat that was always specifying
big-endian.  With this changed to correctly identify the target
endianness, the optimizations work as expected.

I found another case of a call to the same method with big-endian
hardcoded, in PPC::isAllNegativeZeroVector().  I discovered this was
an orphaned method with no callers, so I've just removed it.

The existing test/CodeGen/PowerPC/vec_constants.ll checks these
optimizations, so for testing I've just added a variant for little
endian.

llvm-svn: 234011