Add codegen for VSX word extract conversion from signed/unsigned to single/double
precision.

For UINT_TO_FP:
Extract word unsigned and convert to float was implemented in https://reviews.llvm.org/D20239.
Here we will add the missing extract integer and conversion to double. This
utilizes the new P9 instruction xxextractuw to extracting an integer element
when the result will be converted to double thereby saving 2 direct moves
(VSR <-> GPR).

For SINT_TO_FP:
We will implement the following sequence which will also reduce the number of
instructions by saving 2 direct moves.

v4i32->f32:
        xxspltw
        xvcvsxwsp
        xscvspdpn

v4i32->f64:
        xxspltw
        xvcvsxwdp

Differential Revision: https://reviews.llvm.org/D35859

llvm-svn: 310866