FFI: In absence of externally provided timing, use delay-model as estimate

[hzeller]

Right now, the estimated time for ffi is provided by a delay model, but that is typically not generated for external FFI functions, so it falls back to the ffi_fallback_delay_ps codegen parameter which is an estimate by the user and thus possibly pretty far off. Pipeline schedule will only be as good as the estimate of the parts.

As an alternative, if there is no accurate delay model characterization of the external module, we should use the delay we can determine from the IR for that particular function (as we always have that as fllaback). Even though it is not the same implementation as the external FFI module, it will provide a better estimate assuming similar implementation algorithms.