Function rerun_scc_in_isolation 

fn rerun_scc_in_isolation<'tcx>(graph: &mut ControlFlowGraph<'tcx>, root: usize)

Re-run SCC discovery with a temporarily reduced graph to discover nested SCCs.

Isolates the SCC rooted at root by:

1. Redirecting block 0 to point only to root.
2. Removing back edges to root.
3. Cutting all outgoing edges from SCC exit targets.

After re-running SCC discovery, all edges are restored.