Make execution of declare_vmruntime_limits_ptr conditional again

[frank-emrich]

As of #99, cranelift_wasm::FuncEnvironment::declare_vmruntime_limits_ptr is always executed unconditionally in before_translate_function. This is to ensure that the ir::Variable named vmruntime_limits_ptr can always be used in translate_resume. It would be better to check if there is a resume instruction in the function under consideration, and make the execution of declare_vmruntime_limits_ptr dependent on that.

However, the current workaround should have no runtime overhead, as it simply adds a CLIF load that is potentially unused and therefore eliminated if we don't actually translate a resume.