[Transforms] Fix -Wunused-variable and remove redundant VerifyStates after #75826 (NFC)

llvm-project/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp:1064:18: error: unused variable 'I' [-Werror,-Wunused-variable]
    Instruction *I = cast<Instruction>(Pair.first);
                 ^
llvm-project/llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp:1066:11: error: unused variable 'BaseValue' [-Werror,-Wunused-variable]
    auto *BaseValue = State.getBaseValue();
          ^
2 errors generated.