Function compute_flattening

fn compute_flattening<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: TypingEnv<'tcx>,
    body: &mut Body<'tcx>,
    escaping: BitSet<Local>,
) -> ReplacementMap<'tcx>

Compute the replacement of flattened places into locals.

For each eligible place, we assign a new local to each accessed field. The replacement will be done later in ReplacementVisitor.