Contracts: Only exec parsed code in benchmarks (#3915)

[Weights
compare](https://weights.tasty.limo/compare?unit=weight&ignore_errors=true&threshold=10&method=asymptotic&repo=polkadot-sdk&old=master&new=pg%2Fbench_tweaks&path_pattern=substrate%2Fframe%2F**%2Fsrc%2Fweights.rs%2Cpolkadot%2Fruntime%2F*%2Fsrc%2Fweights%2F**%2F*.rs%2Cpolkadot%2Fbridges%2Fmodules%2F*%2Fsrc%2Fweights.rs%2Ccumulus%2F**%2Fweights%2F*.rs%2Ccumulus%2F**%2Fweights%2Fxcm%2F*.rs%2Ccumulus%2F**%2Fsrc%2Fweights.rs)

Note: Raw weights change does not mean much here, as this PR reduce the
scope of what is benchmarked, they are therefore decreased by a good
margin. One should instead print the Schedule using

cargo test --features runtime-benchmarks bench_print_schedule --
--nocapture
or following the instructions from the
[README](https://github.com/paritytech/polkadot-sdk/tree/pg/bench_tweaks/substrate/frame/contracts#schedule)
for looking at the Schedule of a specific runtime

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Co-authored-by: command-bot <>