[PowerPC] Handle v2i64 comparisons

v2i64 is a legal type under VSX, however we don't have native vector
comparisons. We can handle eq/ne by casting it to an Altivec type, but
everything else must be expanded.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205106 91177308-0d34-0410-b5e6-96231b3b80d8