OpMap: Fix StateMachine issue.

When we are in the CoreOp or Intrinsic state and are about to transition
to the DialectOp state, then we cannot just access the
DialectOpIteratorT in the internal variant storage, since the new
iterator (m_dialectOps.begin()) is not stored in the variant yet.

include/llvm-dialects/Dialect/OpMap.h

@@ -500,6 +500,13 @@ template <typename ValueT, bool isConst> class OpMapIteratorBase final {
       }
 
       if (nextState == IteratorState::DialectOp) {
+        // If we are about to transition to a new state, we cannot assume that
+        // the call to std::get will succeed.
+        if (currentState != nextState)
+          return !m_iterator.m_map->m_dialectOps.empty()
+                     ? IteratorState::DialectOp
+                     : IteratorState::Invalid;
+
         auto peek = std::get<DialectOpIteratorT>(m_iterator.m_iterator);
         std::advance(peek, 1);
         if (peek != m_iterator.m_map->m_dialectOps.end())