Add a new utility to mark a new block as reachable in DeadEndBlocks

Author: meg-gupta

include/swift/SIL/BasicBlockUtils.h

@@ -91,8 +91,11 @@ class DeadEndBlocks {
   /// the set of reachable blocks.
   void updateForReachableBlock(SILBasicBlock *reachableBB);
 
+  /// Add new blocks to the set of reachable blocks.
+  void updateForNewBlock(SILBasicBlock *newBB);
+
   const SILFunction *getFunction() const { return f; }
-  
+
   /// Performs a simple check if \p block (or its single successor) ends in an
   /// "unreachable".
   ///

lib/SIL/Utils/BasicBlockUtils.cpp

@@ -404,6 +404,16 @@ void DeadEndBlocks::updateForReachableBlock(SILBasicBlock *reachableBB) {
   propagateNewlyReachableBlocks(numReachable);
 }
 
+void DeadEndBlocks::updateForNewBlock(SILBasicBlock *newBB) {
+  if (!didComputeValue)
+    return;
+
+  assert(reachableBlocks.count(newBB) == 0);
+  unsigned numReachable = reachableBlocks.size();
+  reachableBlocks.insert(newBB);
+  propagateNewlyReachableBlocks(numReachable);
+}
+
 bool DeadEndBlocks::triviallyEndsInUnreachable(SILBasicBlock *block) {
   // Handle the case where a single "unreachable" block (e.g. containing a call
   // to fatalError()), is jumped to from multiple source blocks.