[PowerPC] Make use of VSX f64 <-> i64 conversion instructions

When VSX is available, these instructions should be used in preference to the
older variants that only have access to the scalar floating-point registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204559 91177308-0d34-0410-b5e6-96231b3b80d8