Currently, when an intrinsic with a fallback body gets used, we always monomorphize and codegen that body: mono item collection does not know whether the backend will invoke the fallback body or supply its own implementation. This has led to perf regressions in #148478 and possibly elsewhere.

It would be better if the backend could inform mono item collection about intrinsics it will overwrite anyway, so that we can avoid codegen'ing those.