fixes based on Steve's comments

stan/math/rev/mat/fun/cholesky_decompose.hpp

@@ -323,16 +323,14 @@ class cholesky_opencl : public vari {
    */
   virtual void chain() {
     int packed_size = M_ * (M_ + 1) / 2;
-    std::vector<double> L_adj_cpu(packed_size);
-    std::vector<double> L_val_cpu(packed_size);
+    std::vector<double> L_adj_cpu;
+    L_adj_cpu.reserve(packed_size);
+    std::vector<double> L_val_cpu;
+    L_val_cpu.reserve(packed_size);
 
-    size_t pos = 0;
-    for (size_type j = 0; j < M_; ++j) {
-      for (size_type i = j; i < M_; ++i) {
-        L_adj_cpu[pos] = vari_ref_L_[pos]->adj_;
-        L_val_cpu[pos] = vari_ref_L_[pos]->val_;
-        ++pos;
-      }
+    for (size_type j = 0; j < packed_size; ++j) {
+        L_adj_cpu[j] = vari_ref_L_[j]->adj_;
+        L_val_cpu[j] = vari_ref_L_[j]->val_;
     }
     matrix_cl L(M_, M_);
     matrix_cl L_adj(M_, M_);
@@ -389,12 +387,9 @@ class cholesky_opencl : public vari {
       L_adj.sub_block(C_adj, 0, 0, k, j, m_k_ind, k_j_ind);
     }
     packed_copy<TriangularViewCL::Lower>(L_adj_cpu, L_adj);
-    pos = 0;
-    for (size_type j = 0; j < M_; ++j)
-      for (size_type i = j; i < M_; ++i) {
-        vari_ref_A_[pos]->adj_ += L_adj_cpu[pos];
-        pos++;
-      }
+    for (size_type j = 0; j < packed_size; ++j) {
+        vari_ref_A_[j]->adj_ += L_adj_cpu[j];
+    }
   }
 };
 #endif