!7750 Add a simplification pattern to GraphKernel's ArithSimplify.

Merge pull request !7750 from DeshiChen/1026_simplify_mul

mindspore/ccsrc/backend/optimizer/graph_kernel/arithmetic_simplify.cc

@@ -258,6 +258,11 @@ AnfNodePtr SimplifyMul(const AnfNodePtr &node) {
     auto new_cnode = NewCNodeWithInfo({NewValueNode(prim::kPrimMul), new_lhs, new_rhs}, node);
     return new_cnode;
   };
+  auto const_dup_lambda2 = [&node, &x, &const_1, &const_2]() -> AnfNodePtr {
+    auto new_rhs = const_1.MulByPatternConst(const_2, x.GetNode(node));
+    auto new_cnode = NewCNodeWithInfo({NewValueNode(prim::kPrimMul), x.GetNode(node), new_rhs}, node);
+    return new_cnode;
+  };
   auto exp_merge_lambda = [&node, &x, &y]() -> AnfNodePtr {
     auto node_tmp = NewCNodeWithInfo({NewValueNode(prim::kPrimTensorAdd), x.GetNode(node), y.GetNode(node)}, node);
     auto new_cnode = NewCNodeWithInfo({NewValueNode(prim::kPrimExp), node_tmp}, node);
@@ -283,6 +288,8 @@ AnfNodePtr SimplifyMul(const AnfNodePtr &node) {
   };
   // (x*C1)*(y*C2) ==> (x*y)*(C1*C2)
   MATCH_REPLACE_LAMBDA(node, (const_1 * x) * (const_2 * y), const_dup_lambda);
+  // (x*C1)*C2 ==> x*(C1*C2)
+  MATCH_REPLACE_LAMBDA(node, (const_1 * x) * const_2, const_dup_lambda2);
   // exp(x)*exp(y) ==> exp(x+y)
   MATCH_REPLACE_LAMBDA(node, PUnaryOperation(prim::kPrimExp, x) * PUnaryOperation(prim::kPrimExp, y), exp_merge_lambda);
   // sqrt(x)*sqrt(x) ==> x