Function rustc_mir::transform::simplify::make_local_map

fn make_local_map<'tcx, V>(
    local_decls: &mut IndexVec<Local, V>, 
    used_locals: &UsedLocals<'tcx>, 
    arg_count: u32
) -> IndexVec<Local, Option<Local>>

Construct the mapping while swapping out unused stuff out from the vec.