[PowerPC] Fix the VSX v2f64 return register

v2f64 values, like other 128-bit values, are returned under VSX in register
vs34 (Altivec register v2).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204543 91177308-0d34-0410-b5e6-96231b3b80d8