[AMDGPU] Fixed build warning

Summary:
V2: Use cast instead of extra if.

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

Differential Revision: https://reviews.llvm.org/D47426

Change-Id: I6ac31da0306f79706960284a7ebd7b9c6237a83a
llvm-svn: 333397