[llvm] [AMDGPU] Add scheduling stage to rewrite MFMA from VGPR to AGPR (PR #170335)

[Matt Arsenault via llvm-commits]

================
@@ -1216,6 +1223,107 @@ bool GCNSchedStage::initGCNSchedStage() {
   return true;
 }
 
+void RewriteScheduleStage::findReachingDefs(
+    MachineOperand &UseMO, LiveIntervals *LIS,
+    SmallVectorImpl<SlotIndex> &DefIdxs) {
+  assert(UseMO.isReg());
+  MachineInstr *UseMI = UseMO.getParent();
+  LiveInterval &UseLI = LIS->getInterval(UseMO.getReg());
+  VNInfo *VNI = UseLI.getVNInfoAt(LIS->getInstructionIndex(*UseMI));
+
+  // If the def is not a PHI, then it must be the only reaching def.
+  if (!VNI->isPHIDef()) {
+    DefIdxs.push_back(VNI->def);
+    return;
+  }
+
+  SmallPtrSet<MachineBasicBlock *, 8> Visited;
+  SmallVector<MachineBasicBlock *, 8> Worklist;
+
+  Visited.insert(UseMI->getParent());
----------------
arsenm wrote:

```suggestion
  SmallPtrSet<MachineBasicBlock *, 8> Visited = {UseMI->getParent()};
  SmallVector<MachineBasicBlock *, 8> Worklist;
```

https://github.com/llvm/llvm-project/pull/170335