[InstSimplify] Fix unused variable warning (NFC)

GCC warning:
```
/llvm-project/llvm/lib/Analysis/InstructionSimplify.cpp: In function ‘llvm::Value* SimplifyWithOpReplaced(llvm::Value*, llvm::Value*, llvm::Value*, const llvm::SimplifyQuery&, bool, unsigned int)’:
/llvm-project/llvm/lib/Analysis/InstructionSimplify.cpp:3993:15: warning: unused variable ‘SI’ [-Wunused-variable]
 3993 |     if (auto *SI = dyn_cast<SelectInst>(I))
      |               ^~
```