[FIRRTL] Fix iteration over set

Fix incorrect iteration over a set to do operation erasure.  The order of
operations is operations is important to avoid erasing an op before all
its users are erased.  Avoid this with a SetVector.

h/t @dtzSiFive for identifying the problem/fix.

Signed-off-by: Schuyler Eldridge <[email protected]>

lib/Dialect/FIRRTL/Transforms/LowerMemory.cpp

@@ -132,7 +132,7 @@ struct LowerMemoryPass
   std::map<FirMemory, FMemModuleOp> memories;
 
   /// A sequence of operations that should be erased later.
-  SmallPtrSet<Operation *, 32> operationsToErase;
+  SetVector<Operation *> operationsToErase;
 };
 } // end anonymous namespace