dpl: fix topological sort cycle with multi-height cells in shift legalizer

Multi-height cells appear in segments across multiple rows. When a
single-height cell sits inside the row span of two multi-height cells,
the ordering differs per row, creating a cycle in the topological sort
which triggers DPL-0400.

Fix: only add dependency edges when both cells' bottom Y matches the
current segment's row bottom, ensuring each multi-height cell
contributes edges only from its primary (bottom) row.

Fixes #9932

Signed-off-by: Divinesoumyadip <soumyacode7@gmail.com>

Author: Divinesoumyadip

src/dpl/src/legalize_shift.cxx

@@ -120,10 +120,18 @@ bool ShiftLegalizer::legalize(DetailedMgr& mgr)
   outgoing_.resize(size);
   for (size_t i = 0; i < mgr.getNumSegments(); i++) {
     const int segId = mgr.getSegment(i)->getSegId();
+    const int rowId = mgr.getSegment(i)->getRowId();
+    const DbuY rowBottom{arch_->getRow(rowId)->getBottom()};
     for (size_t j = 1; j < mgr.getNumCellsInSeg(segId); j++) {
       const Node* prev = mgr.getCellsInSeg(segId)[j - 1];
       const Node* curr = mgr.getCellsInSeg(segId)[j];
 
+      // Only add edges from a cell's bottom (primary) row to avoid
+      // cycles in the topological sort caused by multi-height cells
+      // appearing in multiple row segments with conflicting orderings.
+      if (prev->getBottom() != rowBottom || curr->getBottom() != rowBottom) {
+        continue;
+      }
       incoming_[curr->getId()].push_back(prev->getId());
       outgoing_[prev->getId()].push_back(curr->getId());
     }