Use constexpr members in ConstantSymNodeImpl (pytorch#110142)

A simple refactoring.

Pull Request resolved: pytorch#110142
Approved by: https://github.com/Skylion007

Author: cyyever

c10/core/ConstantSymNodeImpl.h

@@ -18,10 +18,10 @@ class C10_API ConstantSymNodeImpl : public SymNodeImpl {
   ConstantSymNodeImpl(T val) : value_(val) {}
 
   bool is_int() override {
-    return std::is_same<T, int64_t>::value;
+    return is_int_();
   }
   bool is_bool() override {
-    return std::is_same<T, bool>::value;
+    return is_bool_();
   }
   bool is_float() override {
     return false;
@@ -55,21 +55,21 @@ class C10_API ConstantSymNodeImpl : public SymNodeImpl {
   c10::SymNode lt(const c10::SymNode& other) override;
   c10::SymNode gt(const c10::SymNode& other) override;
   std::string str() override {
-    if (is_int()) {
+    if constexpr (is_int_()) {
       return std::to_string(c10::get<int64_t>(value_));
     } else {
       return c10::get<bool>(value_) ? "true" : "false";
     }
   }
   c10::optional<int64_t> constant_int() override {
-    if (is_int()) {
+    if constexpr (is_int_()) {
       return c10::get<int64_t>(value_);
     } else {
       return c10::nullopt;
     }
   }
   c10::optional<bool> constant_bool() override {
-    if (is_bool()) {
+    if constexpr (is_bool_()) {
       return c10::get<bool>(value_);
     } else {
       return c10::nullopt;
@@ -84,6 +84,13 @@ class C10_API ConstantSymNodeImpl : public SymNodeImpl {
 
  private:
   c10::variant<int64_t, bool> value_;
+
+  static constexpr bool is_int_() {
+    return std::is_same<T, int64_t>::value;
+  }
+  static constexpr bool is_bool_() {
+    return std::is_same<T, bool>::value;
+  }
 };
 
 } // namespace c10