[mlir] Simplify functions with "constexpr if" (NFC)

We can use "constexpr if" to combine the two variants of functions.

Author: kazutakahirata

mlir/include/mlir/IR/OpDefinition.h

@@ -1631,14 +1631,11 @@ using detect_has_verify_region_trait =
 
 /// Verify the given trait if it provides a verifier.
 template <typename T>
-std::enable_if_t<detect_has_verify_trait<T>::value, LogicalResult>
-verifyTrait(Operation *op) {
-  return T::verifyTrait(op);
-}
-template <typename T>
-inline std::enable_if_t<!detect_has_verify_trait<T>::value, LogicalResult>
-verifyTrait(Operation *) {
-  return success();
+LogicalResult verifyTrait(Operation *op) {
+  if constexpr (detect_has_verify_trait<T>::value)
+    return T::verifyTrait(op);
+  else
+    return success();
 }
 
 /// Given a set of traits, return the result of verifying the given operation.
@@ -1649,15 +1646,11 @@ LogicalResult verifyTraits(Operation *op) {
 
 /// Verify the given trait if it provides a region verifier.
 template <typename T>
-std::enable_if_t<detect_has_verify_region_trait<T>::value, LogicalResult>
-verifyRegionTrait(Operation *op) {
-  return T::verifyRegionTrait(op);
-}
-template <typename T>
-inline std::enable_if_t<!detect_has_verify_region_trait<T>::value,
-                        LogicalResult>
-verifyRegionTrait(Operation *) {
-  return success();
+LogicalResult verifyRegionTrait(Operation *op) {
+  if constexpr (detect_has_verify_region_trait<T>::value)
+    return T::verifyRegionTrait(op);
+  else
+    return success();
 }
 
 /// Given a set of traits, return the result of verifying the regions of the

mlir/include/mlir/IR/PatternMatch.h

@@ -273,17 +273,13 @@ class RewritePattern : public Pattern {
   template <typename T>
   using detect_has_initialize = llvm::is_detected<has_initialize, T>;
 
-  /// Initialize the derived pattern by calling its `initialize` method.
+  /// Initialize the derived pattern by calling its `initialize` method if
+  /// available.
   template <typename T>
-  static std::enable_if_t<detect_has_initialize<T>::value>
-  initializePattern(T &pattern) {
-    pattern.initialize();
+  static void initializePattern(T &pattern) {
+    if constexpr (detect_has_initialize<T>::value)
+      pattern.initialize();
   }
-  /// Empty derived pattern initializer for patterns that do not have an
-  /// initialize method.
-  template <typename T>
-  static std::enable_if_t<!detect_has_initialize<T>::value>
-  initializePattern(T &) {}
 
   /// An anchor for the virtual table.
   virtual void anchor();

mlir/include/mlir/Pass/PassOptions.h

@@ -75,16 +75,13 @@ static void printOptionValue(raw_ostream &os, const std::string &str) {
     os << "}";
 }
 template <typename ParserT, typename DataT>
-static std::enable_if_t<has_stream_operator<DataT>::value>
-printOptionValue(raw_ostream &os, const DataT &value) {
-  os << value;
-}
-template <typename ParserT, typename DataT>
-static std::enable_if_t<!has_stream_operator<DataT>::value>
-printOptionValue(raw_ostream &os, const DataT &value) {
-  // If the value can't be streamed, fallback to checking for a print in the
-  // parser.
-  ParserT::print(os, value);
+static void printOptionValue(raw_ostream &os, const DataT &value) {
+  if constexpr (has_stream_operator<DataT>::value)
+    os << value;
+  else
+    // If the value can't be streamed, fallback to checking for a print in the
+    // parser.
+    ParserT::print(os, value);
 }
 } // namespace pass_options