[llvm] [NFC][ScheduleDAG] Add SUnit predecessor set (PR #205171)

[Nathan Corbyn via llvm-commits]

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@@ -268,6 +269,7 @@ class TargetRegisterInfo;
 
     SmallVector<SDep, 4> Preds;  ///< All sunit predecessors.
     SmallVector<SDep, 4> Succs;  ///< All sunit successors.
+    std::unique_ptr<BitVector> PredsSet; ///< Predecessors set by NodeNum.
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cofibrant wrote:

Shouldn't this be a `SparseBitVector`?

https://github.com/llvm/llvm-project/pull/205171