AMDGPU/SI: Remove zero_extend patterns for i16 ops selected to 32-bit insts

Summary:
The 32-bit instructions don't zero the high 16-bits like the 16-bit
instructions do.

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D26828

llvm-svn: 287342