[NFC] Use more appropriate SmallVectorImpl::append call in std::initializer_list SmallVector constructor

[yurai007]

Summary

Since we are in constructor there is no need to perform redundant call to SmallVectorImpl::clear() inside assign function.
Although calling cheaper append function instead assign doesn't make any difference on optimized builds (DSE does the job removing stores), we still save some cycles
for debug binaries.

[nikic]

LGTM

[barannikov88]

LGTM

llvm/include/llvm/ADT/SmallVector.h

public:
 
   template <typename RangeTy>
   explicit SmallVector(const iterator_range<RangeTy> &R)
       : SmallVectorImpl<T>(N) {
     this->append(R.begin(), R.end());
   }
 
   SmallVector(std::initializer_list<T> IL) : SmallVectorImpl<T>(N) {
-    this->assign(IL);
+    this->append(IL);
   }
 
   SmallVector(const SmallVector &RHS) : SmallVectorImpl<T>(N) {
     if (!RHS.empty())
       SmallVectorImpl<T>::operator=(RHS);
   }
 
   SmallVector &operator=(const SmallVector &RHS) {