X86: Improve AVX512 fptoui lowering

Summary:
Add patterns for
  fptoui <16 x float> to <16 x i8>
  fptoui <16 x float> to <16 x i16>

Reviewers: igorb, delena, craig.topper

Reviewed By: craig.topper

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37505

llvm-svn: 312704