[X86] Correct the cost of (v4i32 (fptoui (v4f64))) under AVX512F.

Summary: This was inheriting the cost from the AVX table, but should be legal under AVX512.

Reviewers: RKSimon

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D51267

llvm-svn: 340708