[PowerPC] Keep vector int to fp conversions in vector domain

At present a v2i16 -> v2f64 convert is implemented by extracts to scalar,
scalar converts, and merge back into a vector. Use vector converts instead,
with the int data permuted into the proper position and extended if necessary.

Patch by RolandF.

Differential revision: https://reviews.llvm.org/D53346

llvm-svn: 345361