Fix check for prescheduling to copies from virtual registers

[colpell]

Summary

Currently, this check on line 2834 is an exact copy of a check on line 2810, which SU should have already passed. Given the context and comment above the check ("Avoid prescheduling *to* copies" vs. "Avoid prescheduling copies"), it seems like the intention was to check PredSU instead.

[colpell]

After conversation with Justin, this revision does not appear to be correct.

lib/CodeGen/SelectionDAG/ScheduleDAGRRList.cpp

for (SUnit &SU : *SUnits) {
     // support infrastructure.
     if (PredSU->hasPhysRegDefs)
       continue;
     // Short-circuit the case where SU is PredSU's only data successor.
     if (PredSU->NumSuccs == 1)
       continue;
     // Avoid prescheduling to copies from virtual registers, which don't behave
     // like other nodes from the perspective of scheduling heuristics.
-    if (SDNode *N = SU.getNode())
+    if (SDNode *N = PredSU->getNode())
       if (N->getOpcode() == ISD::CopyFromReg &&
           TargetRegisterInfo::isVirtualRegister
             (cast<RegisterSDNode>(N->getOperand(1))->getReg()))
         continue;
 
     // Perform checks on the successors of PredSU.
     for (const SDep &PredSucc : PredSU->Succs) {
       SUnit *PredSuccSU = PredSucc.getSUnit();