AMDGPU/SI: Promote i16 = fp_[us]int f32 for VI

Summary: This fixes a regression caused by r286464.

Reviewers: arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, llvm-commits, tony-tye

Differential Revision: https://reviews.llvm.org/D26570

llvm-svn: 286687