[InstCombine] Remove unused variables in InstCombineSelect.cpp (NFC)

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:3810:14: error: unused variable 'LHS' [-Werror,-Wunused-variable]
 3810 |       Value *LHS, *RHS;
      |              ^~~
llvm-project/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp:3810:20: error: unused variable 'RHS' [-Werror,-Wunused-variable]
 3810 |       Value *LHS, *RHS;
      |