[amdgpu] Fix check of VCC.

Summary: - Need to include checking on the new 16-bit subregs.

Reviewers: rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79498