[mlir][scf] fix getSuccessorRegions func in scf.forall (#147491)

In accordance with the semantics of forall, its body is executed in
parallel by multiple threads. We should not expect to branch back into
the forall body after the region's execution is complete.

Author: cxy-1993

mlir/lib/Dialect/SCF/IR/SCF.cpp

@@ -1926,11 +1926,13 @@ void ForallOp::getCanonicalizationPatterns(RewritePatternSet &results,
 /// not a constant.
 void ForallOp::getSuccessorRegions(RegionBranchPoint point,
                                    SmallVectorImpl<RegionSuccessor> &regions) {
-  // Both the operation itself and the region may be branching into the body or
-  // back into the operation itself. It is possible for loop not to enter the
-  // body.
-  regions.push_back(RegionSuccessor(&getRegion()));
-  regions.push_back(RegionSuccessor());
+  // In accordance with the semantics of forall, its body is executed in
+  // parallel by multiple threads. We should not expect to branch back into
+  // the forall body after the region's execution is complete.
+  if (point.isParent())
+    regions.push_back(RegionSuccessor(&getRegion()));
+  else
+    regions.push_back(RegionSuccessor());
 }
 
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