v2[fi]64 values need to be explicitly passed in VSX registers. This is because
the code in TRI that finds the minimal register class given a register and a
value type will assert if given an Altivec register and a non-Altivec type.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205041 91177308-0d34-0410-b5e6-96231b3b80d8
