[clang][amdgpu] Prefer not using `fp16` conversion intrinsics.

Reviewers: yaxunl, arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, dstuttard, tpr, t-tye, kerbowa, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81849