[InstCombine][AMDGPU] Fix crash with v3i16/v3f16 buffer intrinsics

Summary:
This is something of a workaround to avoid a crash later on in type
legalizer (WidenVectorResult()).
Also added some f16 tests, including a non-working v3f16 case with
a FIXME.

Reviewers: arsenm, tpr, nhaehnle

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68865

llvm-svn: 374993