[x86] allow narrowing of vector UINT_TO_FP

As discussed in:
D56864
D58197

Always use the narrow (128-bit) instruction when possible.
We already had the signed int version of this transform.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354675 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed