[mono][interpreter] Fix performance of new span helpers

https://github.com/dotnet/runtime/pull/73768 regressed Mono. It was "reverted" in https://github.com/dotnet/runtime/pull/75917 to work around the regression but largely fixed for AOT in https://github.com/dotnet/runtime/pull/76033 but we cannot currently remove the workaround because the interpreter is still impacted.