Update XLS_VLOG references in docs.

PiperOrigin-RevId: 617978474

Author: grebe

docs_src/optimizations/optimizations.md

@@ -117,7 +117,7 @@ original leaf node A is being removed and a corresponding logging statement
         }
       }
     }
-    XLS_VLOG(3) << "DCE removing " << node->ToString();
+    VLOG(3) << "DCE removing " << node->ToString();
     XLS_RETURN_IF_ERROR(f->RemoveNode(node));
     removed_count++;
   }
@@ -128,7 +128,7 @@ For this pass, this amounts to returning whether or not a single IR node has
 been DCE'ed:
 
 ```c++
-  XLS_VLOG(2) << "Removed " << removed_count << " dead nodes";
+  VLOG(2) << "Removed " << removed_count << " dead nodes";
   return removed_count > 0;
 }
 ```
@@ -349,7 +349,7 @@ and mark the IR as modified. We also note whether a true match was found (via
 `replaced`):
 
 ```c++
-        XLS_VLOG(3) << absl::StreamFormat(
+        VLOG(3) << absl::StreamFormat(
             "Replacing %s with equivalent node %s", node->GetName(),
             candidate->GetName());
         XLS_RETURN_IF_ERROR(node->ReplaceUsesWith(candidate));
@@ -416,7 +416,7 @@ literals as operands as well as whether it is safe to replace the node.
         !OpIsSideEffecting(node->op()) &&
         std::all_of(node->operands().begin(), node->operands().end(),
                     [](Node* o) { return o->Is<Literal>(); })) {
-      XLS_VLOG(2) << "Folding: " << *node;
+      VLOG(2) << "Folding: " << *node;
 ```
 
 Here now comes the fun part. If the condition is true, XLS simply collects the