Add custom UINT_TO_FP from v4i8/v4i16/v8i8/v8i16 to v4f32/v8f32

- Replace v4i8/v8i8 -> v8f32 DAG combine with custom lowering to reduce
  DAG combine overhead.
- Extend the support to v4i16/v8i16 as well.

llvm-svn: 166487