Fix an edge case in JointPostDominanceSetComputer::findJointPostDominatingSet

While computing blocksThatLeakIfNeverVisited by processing the worklist,
we should also add all the successors of a predecessor block that was in
the dominatedBlockSet initially. If not we will end up with incorrect
result for a control flow like this :

dominating block : bb1
dominated block : bb2

                         +----+
                         | bb1|
                         +----+
             +------------>|
             |             v
             |           +-+--+
             |           | bb2| -----+
             |           +-+--+      |
             |             |         |
             |             v         v
             |           +-+--+    +-+--+
             |           |bb3 |    | bb4|
             |           +--+-+    +----+
             |              |
             |              |
             +--------------+

Without the fix blocksThatLeakIfNeverVisited will not have bb4.

The sil test for this fix is rle_redundantload_does_not_postdominate2
in redundant_load_elim_ossa_complex.sil

Author: meg-gupta

lib/SIL/Utils/BasicBlockUtils.cpp

@@ -463,6 +463,13 @@ void JointPostDominanceSetComputer::findJointPostDominatingSet(
     for (auto *predBlock : block->getPredecessorBlocks()) {
       if (initialBlocks.count(predBlock)) {
         reachableInputBlocks.push_back(predBlock);
+        for (auto *succBlock : predBlock->getSuccessorBlocks()) {
+          if (visitedBlocks.count(succBlock))
+            continue;
+          if (deadEndBlocks.isDeadEnd(succBlock))
+            continue;
+          blocksThatLeakIfNeverVisited.insert(succBlock);
+        }
       }
       if (visitedBlocks.insert(predBlock).second)
         worklist.push_back(predBlock);