[Hexagon] Construct SmallVector with an iterator range (NFC) (llvm#136416)

This patch switches to SmallVector so that we can construct an
instance with an iterator range.

Author: kazutakahirata

llvm/lib/Target/Hexagon/HexagonLoopIdiomRecognition.cpp

@@ -1079,9 +1079,7 @@ bool PolynomialMultiplyRecognize::promoteTypes(BasicBlock *LoopB,
       return false;
 
   // Perform the promotion.
-  std::vector<Instruction*> LoopIns;
-  std::transform(LoopB->begin(), LoopB->end(), std::back_inserter(LoopIns),
-                 [](Instruction &In) { return &In; });
+  SmallVector<Instruction *> LoopIns(llvm::make_pointer_range(*LoopB));
   for (Instruction *In : LoopIns)
     if (!In->isTerminator())
       promoteTo(In, DestTy, LoopB);