[NFC] Remove some boilerplate from SmallVector header

[yurai007]

Summary

In SmallVector header we use couple of times exactly same enable_if, in this change we de-duplicate it and declare only once.

Extracted from: https://reviews.llvm.org/D129990

[dblaikie]

Sounds good, thanks!

llvm/include/llvm/ADT/SmallVector.h

 #include <new>
 #include <type_traits>
 #include <utility>
 
 namespace llvm {
 
 template <typename IteratorT> class iterator_range;
 
+template <class Iterator>
+using EnableIfConvertibleToInputIterator = std::enable_if_t<std::is_convertible<
+    typename std::iterator_traits<Iterator>::iterator_category,
+    std::input_iterator_tag>::value>;
+
 /// This is all the stuff common to all SmallVectors.
 ///
 /// The template parameter specifies the type which should be used to hold the
 /// Size and Capacity of the SmallVector, so it can be adjusted.
 /// Using 32 bit size is desirable to shrink the size of the SmallVector.
 /// Using 64 bit size is desirable for cases like SmallVector<char>, where a
 /// 32 bit size would limit the vector to ~4GB. SmallVectors are used for
 /// buffering bitcode output - which can exceed 4GB.
LLVM_NODISCARD T pop_back_val() {
     T Result = ::std::move(this->back());
     this->pop_back();
     return Result;
   }
 
   void swap(SmallVectorImpl &RHS);
 
   /// Add the specified range to the end of the SmallVector.
-  template <typename in_iter,
-            typename = std::enable_if_t<std::is_convertible<
-                typename std::iterator_traits<in_iter>::iterator_category,
-                std::input_iterator_tag>::value>>
-  void append(in_iter in_start, in_iter in_end) {
+  template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
+  void append(ItTy in_start, ItTy in_end) {
     this->assertSafeToAddRange(in_start, in_end);
     size_type NumInputs = std::distance(in_start, in_end);
     this->reserve(this->size() + NumInputs);
     this->uninitialized_copy(in_start, in_end, this->end());
     this->set_size(this->size() + NumInputs);
   }
 
   /// Append \p NumInputs copies of \p Elt to the end.
void assign(size_type NumElts, ValueParamT Elt) {
     else if (NumElts < this->size())
       this->destroy_range(this->begin() + NumElts, this->end());
     this->set_size(NumElts);
   }
 
   // FIXME: Consider assigning over existing elements, rather than clearing &
   // re-initializing them - for all assign(...) variants.
 
-  template <typename in_iter,
-            typename = std::enable_if_t<std::is_convertible<
-                typename std::iterator_traits<in_iter>::iterator_category,
-                std::input_iterator_tag>::value>>
-  void assign(in_iter in_start, in_iter in_end) {
+  template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
+  void assign(ItTy in_start, ItTy in_end) {
     this->assertSafeToReferenceAfterClear(in_start, in_end);
     clear();
     append(in_start, in_end);
   }
 
   void assign(std::initializer_list<T> IL) {
     clear();
     append(IL);
iterator insert(iterator I, size_type NumToInsert, ValueParamT Elt) {
     // Replace the overwritten part.
     std::fill_n(I, NumOverwritten, *EltPtr);
 
     // Insert the non-overwritten middle part.
     std::uninitialized_fill_n(OldEnd, NumToInsert - NumOverwritten, *EltPtr);
     return I;
   }
 
-  template <typename ItTy,
-            typename = std::enable_if_t<std::is_convertible<
-                typename std::iterator_traits<ItTy>::iterator_category,
-                std::input_iterator_tag>::value>>
+  template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
   iterator insert(iterator I, ItTy From, ItTy To) {
     // Convert iterator to elt# to avoid invalidating iterator when we reserve()
     size_t InsertElt = I - this->begin();
 
     if (I == this->end()) {  // Important special case for empty vector.
       append(From, To);
       return this->begin()+InsertElt;
     }
~SmallVector() {
     this->destroy_range(this->begin(), this->end());
   }
 
   explicit SmallVector(size_t Size, const T &Value = T())
     : SmallVectorImpl<T>(N) {
     this->assign(Size, Value);
   }
 
-  template <typename ItTy,
-            typename = std::enable_if_t<std::is_convertible<
-                typename std::iterator_traits<ItTy>::iterator_category,
-                std::input_iterator_tag>::value>>
+  template <typename ItTy, typename = EnableIfConvertibleToInputIterator<ItTy>>
   SmallVector(ItTy S, ItTy E) : SmallVectorImpl<T>(N) {
     this->append(S, E);
   }
 
   template <typename RangeTy>
   explicit SmallVector(const iterator_range<RangeTy> &R)
       : SmallVectorImpl<T>(N) {
     this->append(R.begin(), R.end());