[mandatory-combine] Simplify block traversal used to initialize worklist.

Previously, we were using a pointer set to ensure that was checked when a value
was popped off the worklist. In this commit, I change the code so that we
instead use the set to guard insertion into the worklist.

I changed the name of the set to blockAlreadyAddedToWorklist. Hopefully that
will make things a bit clearer.

Author: gottesmm

lib/SILOptimizer/Mandatory/MandatoryCombine.cpp

@@ -127,20 +127,18 @@ class MandatoryCombiner final
 //===----------------------------------------------------------------------===//
 
 void MandatoryCombiner::addReachableCodeToWorklist(SILFunction &function) {
-  SmallBlotSetVector<SILBasicBlock *, 32> blockWorklist;
-  SmallBlotSetVector<SILBasicBlock *, 32> blocksVisited;
-  SmallVector<SILInstruction *, 128> instructions;
+  SmallVector<SILBasicBlock *, 32> blockWorklist;
+  SmallPtrSet<SILBasicBlock *, 32> blockAlreadyAddedToWorklist;
+  SmallVector<SILInstruction *, 128> initialInstructionWorklist;
+
+  {
+    auto *firstBlock = &*function.begin();
+    blockWorklist.push_back(firstBlock);
+    blockAlreadyAddedToWorklist.insert(firstBlock);
+  }
 
-  blockWorklist.insert(&*function.begin());
   while (!blockWorklist.empty()) {
-    auto *block = blockWorklist.pop_back_val().getValueOr(nullptr);
-    if (block == nullptr) {
-      continue;
-    }
-
-    if (!blocksVisited.insert(block).second) {
-      continue;
-    }
+    auto *block = blockWorklist.pop_back_val();
 
     for (auto iterator = block->begin(), end = block->end(); iterator != end;) {
       auto *instruction = &*iterator;
@@ -150,14 +148,17 @@ void MandatoryCombiner::addReachableCodeToWorklist(SILFunction &function) {
         continue;
       }
 
-      instructions.push_back(instruction);
+      initialInstructionWorklist.push_back(instruction);
     }
 
-    for_each(block->getSuccessorBlocks(),
-             [&](SILBasicBlock *block) { blockWorklist.insert(block); });
+    llvm::copy_if(block->getSuccessorBlocks(),
+                  std::back_inserter(blockWorklist),
+                  [&](SILBasicBlock *block) -> bool {
+                    return blockAlreadyAddedToWorklist.insert(block).second;
+                  });
   }
 
-  worklist.addInitialGroup(instructions);
+  worklist.addInitialGroup(initialInstructionWorklist);
 }
 
 bool MandatoryCombiner::doOneIteration(SILFunction &function,