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

GCC warning:
```
/llvm-project/llvm/lib/Analysis/ConstantFolding.cpp: In function ‘llvm::Constant* llvm::ConstantFoldLoadFromConstPtr(llvm::Constant*, llvm::Type*, const llvm::DataLayout&)’:
/llvm-project/llvm/lib/Analysis/ConstantFolding.cpp:713:19: warning: unused variable ‘SimplifiedGEP’ [-Wunused-variable]
  713 |         if (auto *SimplifiedGEP = dyn_cast<GEPOperator>(Simplified)) {
      |                   ^~~~~~~~~~~~~
```