[llvm] [MISched](NFC) Factor control dependency construction state and logic out of `buildSchedGraph` (PR #226533)

[Tomer Shafir via llvm-commits]

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@@ -620,8 +597,29 @@ void ScheduleDAGInstrs::initSUnits() {
   }
 }
 
-class ScheduleDAGInstrs::Value2SUsMap
-    : public SmallMapVector<ValueType, SUList, 4> {
+namespace {
+/// A list of SUnits, used in Value2SUsMap, during DAG construction.
+/// Note: to gain speed it might be worth investigating an optimized
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tomershafir wrote:

If we are here - can we change `Note` to `FIXME`?

https://github.com/llvm/llvm-project/pull/226533