Update Wasm benchmarks (#2957)

In https://github.com/paritytech/polkadot-sdk/pull/2941 we found out
that the new Wasmi (register) is very effective at optimizing away
certain benchmark bytecode constructs in a way that created an unfair
advantage over Wasmi (stack) which yielded our former benchmarks to be
ineffective at properly measuring the performance impact.

This PR adjusts both affected benchmarks to fix the stated problems.
Affected are
- `instr_i64const` -> `instr_i64add`: Renamed since it now measures the
performance impact of the Wasm `i64.add` instruction with locals as
inputs and outputs. This makes it impossible for Wasmi (register) to
aggressively optimize away the entire function body (as it previously
did) but still provides a way for Wasmi (register) to shine with its
register based execution model.
- `call_with_code_per_byte`: Now uses `local.get` instead of `i32.const`
for the `if` condition which prevents Wasmi (register) to aggressively
optimizing away whole parts of the `if` creating an unfair advantage.

cc @athei



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Co-authored-by: command-bot <>
Co-authored-by: Alexander Theißen <[email protected]>
Co-authored-by: Ignacio Palacios <[email protected]>