AMDGPU: Fix UBSan error caused by r335942

Summary: Fixes PR38071.

Reviewers: arsenm, dstenb

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D48979

llvm-svn: 336448