[libc++] Simplify vector<bool>::__construct_at_end (#119632)

This patch simplifies the implementation of `__construct_at_end` in
`vector<bool>`, which currently contains duplicate initialization logic
across its two overloads.

Author: winner245

libcxx/include/__vector/vector_bool.h

@@ -552,36 +552,29 @@ vector<bool, _Allocator>::__recommend(size_type __new_size) const {
 }
 
 //  Default constructs __n objects starting at __end_
-//  Precondition:  __n > 0
 //  Precondition:  size() + __n <= capacity()
 //  Postcondition:  size() == size() + __n
 template <class _Allocator>
 inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 vector<bool, _Allocator>::__construct_at_end(size_type __n, bool __x) {
-  size_type __old_size = this->__size_;
+  _LIBCPP_ASSERT_INTERNAL(
+      capacity() >= size() + __n, "vector<bool>::__construct_at_end called with insufficient capacity");
+  std::fill_n(end(), __n, __x);
   this->__size_ += __n;
-  if (__old_size == 0 || ((__old_size - 1) / __bits_per_word) != ((this->__size_ - 1) / __bits_per_word)) {
-    if (this->__size_ <= __bits_per_word)
-      this->__begin_[0] = __storage_type(0);
-    else
-      this->__begin_[(this->__size_ - 1) / __bits_per_word] = __storage_type(0);
-  }
-  std::fill_n(__make_iter(__old_size), __n, __x);
+  if (end().__ctz_ != 0) // Ensure uninitialized leading bits in the last word are set to zero
+    std::fill_n(end(), __bits_per_word - end().__ctz_, 0);
 }
 
 template <class _Allocator>
 template <class _InputIterator, class _Sentinel>
 _LIBCPP_CONSTEXPR_SINCE_CXX20 void
 vector<bool, _Allocator>::__construct_at_end(_InputIterator __first, _Sentinel __last, size_type __n) {
-  size_type __old_size = this->__size_;
+  _LIBCPP_ASSERT_INTERNAL(
+      capacity() >= size() + __n, "vector<bool>::__construct_at_end called with insufficient capacity");
+  std::__copy(std::move(__first), std::move(__last), end());
   this->__size_ += __n;
-  if (__old_size == 0 || ((__old_size - 1) / __bits_per_word) != ((this->__size_ - 1) / __bits_per_word)) {
-    if (this->__size_ <= __bits_per_word)
-      this->__begin_[0] = __storage_type(0);
-    else
-      this->__begin_[(this->__size_ - 1) / __bits_per_word] = __storage_type(0);
-  }
-  std::__copy(std::move(__first), std::move(__last), __make_iter(__old_size));
+  if (end().__ctz_ != 0) // Ensure uninitialized leading bits in the last word are set to zero
+    std::fill_n(end(), __bits_per_word - end().__ctz_, 0);
 }
 
 template <class _Allocator>