[AMDGPU] Cluster export instruction in PostRA Scheduler

DAG mutation needs to be applied post-RA to maintain order
established during pre-RA scheduler.

Author: perlfu

llvm/lib/Target/AMDGPU/AMDGPUTargetMachine.cpp

@@ -1129,6 +1129,7 @@ GCNTargetMachine::createPostMachineScheduler(MachineSchedContext *C) const {
        getOptLevel() >= CodeGenOptLevel::Less) &&
       EnableVOPD)
     DAG->addMutation(createVOPDPairingMutation());
+  DAG->addMutation(createAMDGPUExportClusteringDAGMutation());
   return DAG;
 }
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