[SCEV] Fix unused variable warning (NFC)

Author: nikic

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -8792,7 +8792,7 @@ static Constant *BuildConstantFromSCEV(const SCEV *V) {
  if (C2->getType()->isPointerTy())
  return nullptr;
 
- if (PointerType *PTy = dyn_cast<PointerType>(C->getType())) {
+ if (C->getType()->isPointerTy()) {
  C = ConstantExpr::getGetElementPtr(Type::getInt8Ty(C->getContext()),
  C, C2);
  } else {