[AutoDiff] Minor activity analysis changes. (#28301)

Hoist activity marking visited value set out of loop over original bbs.
This is safe because bbs directly start with dominator bbs's active values.

Visit bb arguments for activity marking. This was accidentally deleted in
#28225. Re-adding the logic doesn't
seem to affect any tests.

Author: dan-zheng

lib/SILOptimizer/Mandatory/Differentiation.cpp

@@ -6231,6 +6231,8 @@ class PullbackEmitter final : public SILInstructionVisitor<PullbackEmitter> {
     // Adjoint values of dominated active values are passed as pullback block
     // arguments.
     DominanceOrder domOrder(original.getEntryBlock(), domInfo);
+    // Keep track of visited values.
+    SmallPtrSet<SILValue, 8> visited;
     while (auto *bb = domOrder.getNext()) {
       auto &bbActiveValues = activeValues[bb];
       // If the current block has an immediate dominator, append the immediate
@@ -6240,13 +6242,12 @@ class PullbackEmitter final : public SILInstructionVisitor<PullbackEmitter> {
         bbActiveValues.append(domBBActiveValues.begin(),
                               domBBActiveValues.end());
       }
-      SmallPtrSet<SILValue, 8> visited(bbActiveValues.begin(),
-                                       bbActiveValues.end());
-      // Register a value as active if it has not yet been visited.
       bool diagnosedActiveEnumValue = false;
-      auto addActiveValue = [&](SILValue v) {
+      // Mark the activity of a value if it has not yet been visited.
+      auto markValueActivity = [&](SILValue v) {
         if (visited.count(v))
           return;
+        visited.insert(v);
         // Diagnose active enum values. Differentiation of enum values requires
         // special adjoint value handling and is not yet supported. Diagnose
         // only the first active enum value to prevent too many diagnostics.
@@ -6262,17 +6263,19 @@ class PullbackEmitter final : public SILInstructionVisitor<PullbackEmitter> {
         // become projections into their adjoint base buffer.
         if (Projection::isAddressProjection(v))
           return;
-        visited.insert(v);
         bbActiveValues.push_back(v);
       };
-      // Register bb arguments and all instruction operands/results.
+      // Visit bb arguments and all instruction operands/results.
+      for (auto *arg : bb->getArguments())
+        if (getActivityInfo().isActive(arg, getIndices()))
+          markValueActivity(arg);
       for (auto &inst : *bb) {
         for (auto op : inst.getOperandValues())
           if (getActivityInfo().isActive(op, getIndices()))
-            addActiveValue(op);
+            markValueActivity(op);
         for (auto result : inst.getResults())
           if (getActivityInfo().isActive(result, getIndices()))
-            addActiveValue(result);
+            markValueActivity(result);
       }
       domOrder.pushChildren(bb);
       if (errorOccurred)