codegen-backend invokes collector twice

[nikomatsakis]

This code looks like it is invoking the collector twice (link):

fn codegen_crate<'a, 'tcx>(
        &self,
        tcx: TyCtxt<'a, 'tcx, 'tcx>,
        _rx: mpsc::Receiver<Box<dyn Any + Send>>
) -> Box<dyn Any> {
    ...
    ::rustc_mir::monomorphize::assert_symbols_are_distinct(tcx,
            collector::collect_crate_mono_items(
                tcx,
                collector::MonoItemCollectionMode::Eager
            ).0.iter()
    ...
    for mono_item in
            collector::collect_crate_mono_items(
                tcx,
                collector::MonoItemCollectionMode::Eager
            ).0 {
            ...
    }
}

Per a quick conversation on Zulip, it seems like this is a bug. It should probably be invoking the collect_and_partition_mono_items query.

cc @michaelwoerister @eddyb @irinagpopa