[PowerPC] Complete the custom legalization of vector int to fp conversion

A recent patch has added custom legalization of vector conversions of
v2i16 -> v2f64. This just rounds it out for other types where the input vector
has an illegal (narrower) type than the result vector. Specifically, this will
handle the following conversions:

v2i8 -> v2f64
v4i8 -> v4f32
v4i16 -> v4f32

Differential revision: https://reviews.llvm.org/D54663

llvm-svn: 350155