Internals: Simplify DfgPeephole replacement scheme

Always only replace the currently iterated node. This ensures it is not
on the work list, which simplifies upcoming work.

No functional of performance change intended.

src/V3DfgCache.h

@@ -339,7 +339,7 @@ public:
         for (DfgVertex& vtx : m_dfg.opVertices()) cache(&vtx);
     }
 
-    // Add an existing vertex to the cache. If an equivalent already exists,
+    // Add an existing vertex to the cache. If an equivalent (but different) already exists,
     // it is returned and the cache is not updated.
     DfgVertex* cache(DfgVertex* vtxp) { return m_vtxType2Cachep[vtxp->type()]->cache(vtxp); }