[AMDGPU] Cluster shader exports

Summary:
Add DAG scheduling mutation to cluster export instructions.
This avoids unnecessary waitcnts being added when computation
ends up interspersed with exports.

Reviewers: foad, arsenm, rampitec, nhaehnle

Reviewed By: foad

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79481