[AMDGPU] Add VerifyScheduling support.

Summary:
This is cut and pasted from the corresponding GenericScheduler
functions.

Reviewers: arsenm, atrick, tstellar, vpykhtin

Subscribers: MatzeB, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, javed.absar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68264

llvm-svn: 373346