[mlir] allow overriding visitRegionSuccessors (llvm#125268)

This PR enables overriding AbstractSparseForwardDataFlowAnalysis::visitRegionSuccessors to control precisely how the region successors of `branch` are visited. For example in order to precisely control the order in which predecessor operand lattices are propagated from. An override is responsible for visiting all the known predecessors and propagating therefrom.

Author: makslevental

mlir/include/mlir/Analysis/DataFlow/SparseAnalysis.h

@@ -255,10 +255,15 @@ class AbstractSparseForwardDataFlowAnalysis : public DataFlowAnalysis {
   /// operation `branch`, which can either be the entry block of one of the
   /// regions or the parent operation itself, and set either the argument or
   /// parent result lattices.
-  void visitRegionSuccessors(ProgramPoint *point,
-                             RegionBranchOpInterface branch,
-                             RegionBranchPoint successor,
-                             ArrayRef<AbstractSparseLattice *> lattices);
+  /// This method can be overridden to control precisely how the region
+  /// successors of `branch` are visited. For example in order to precisely
+  /// control the order in which predecessor operand lattices are propagated
+  /// from. An override is responsible for visiting all the known predecessors
+  /// and propagating therefrom.
+  virtual void
+  visitRegionSuccessors(ProgramPoint *point, RegionBranchOpInterface branch,
+                        RegionBranchPoint successor,
+                        ArrayRef<AbstractSparseLattice *> lattices);
 };
 
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