Snoop: improve accuracy of N_IContract_const

The contract address for a benchmark is randomly generated at https://gitlab.com/tezos/tezos/-/blob/master/src/proto_alpha/lib_benchmark/michelson_samplers.ml#L508-519 . Currently, gas for CONTRACT is calculated as the average of both types of contract address, but the CONTRACT operation for an originated account is about 5 times slower than for an implicit account.