[csa] Return last value from BuildFastLoop to allow chaining sequential loops

Review-Url: https://codereview.chromium.org/2657293002
Cr-Commit-Position: refs/heads/master@{#42774}

src/code-stub-assembler.cc

@@ -6258,7 +6258,7 @@ Node* CodeStubAssembler::CreateWeakCellInFeedbackVector(Node* feedback_vector,
   return cell;
 }
 
-void CodeStubAssembler::BuildFastLoop(
+Node* CodeStubAssembler::BuildFastLoop(
     const CodeStubAssembler::VariableList& vars,
     MachineRepresentation index_rep, Node* start_index, Node* end_index,
     const FastLoopBody& body, int increment, IndexAdvanceMode mode) {
@@ -6287,6 +6287,7 @@ void CodeStubAssembler::BuildFastLoop(
     Branch(WordNotEqual(var.value(), end_index), &loop, &after_loop);
   }
   Bind(&after_loop);
+  return var.value();
 }
 
 void CodeStubAssembler::BuildFastFixedArrayForEach(

src/code-stub-assembler.h

@@ -1075,16 +1075,16 @@ class V8_EXPORT_PRIVATE CodeStubAssembler : public compiler::CodeAssembler {
 
   typedef std::function<void(Node* index)> FastLoopBody;
 
-  void BuildFastLoop(const VariableList& var_list,
+  Node* BuildFastLoop(const VariableList& var_list,
                       MachineRepresentation index_rep, Node* start_index,
                       Node* end_index, const FastLoopBody& body, int increment,
                       IndexAdvanceMode mode = IndexAdvanceMode::kPre);
 
-  void BuildFastLoop(MachineRepresentation index_rep, Node* start_index,
+  Node* BuildFastLoop(MachineRepresentation index_rep, Node* start_index,
                       Node* end_index, const FastLoopBody& body, int increment,
                       IndexAdvanceMode mode = IndexAdvanceMode::kPre) {
-    BuildFastLoop(VariableList(0, zone()), index_rep, start_index, end_index,
-                  body, increment, mode);
+    return BuildFastLoop(VariableList(0, zone()), index_rep, start_index,
+                         end_index, body, increment, mode);
   }
 
   enum class ForEachDirection { kForward, kReverse };