[SCEV] Fix -Wunused-variable warning (NFC)

GCC warning:
```
/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp: In member function ‘const llvm::SCEV* llvm::ScalarEvolution::getLosslessPtrToIntExpr(const llvm::SCEV*, unsigned int)::SCEVPtrToIntSinkingRewriter::visitUnknown(const llvm::SCEVUnknown*)’:
/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:1152:13: warning: unused variable ‘ExprPtrTy’ [-Wunused-variable]
 1152 |       Type *ExprPtrTy = Expr->getType();
      |             ^~~~~~~~~
```