[PowerPC] v2[fi]64 need to be explicitly passed in VSX registers

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.
Convert one last llc -filetype=obj test.

Unfortunately this one fails deep inside the mips backend, so xfail it.