CodeGen: BlockPlacement: Add comment about DenseMap Safety.

The use of a DenseMap in precomputeTriangleChains does not cause
non-determinism, even though it is iterated over, as the only thing the
iteration does is to insert entries into a new DenseMap, which is not iterated.
Comment only change.

llvm-svn: 300088

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -1165,6 +1165,9 @@ void MachineBlockPlacement::precomputeTriangleChains() {
     }
   }
 
+  // Iterating over a DenseMap is safe here, because the only thing in the body
+  // of the loop is inserting into another DenseMap (ComputedEdges).
+  // ComputedEdges is never iterated, so this doesn't lead to non-determinism.
   for (auto &ChainPair : TriangleChainMap) {
     TriangleChain &Chain = ChainPair.second;
     // Benchmarking has shown that due to branch correlation duplicating 2 or