Function cleanup_identities 

pub fn cleanup_identities<'a>(
    stmts: Vec<Stmt<'a>>,
    expr_arena: &'a Arena<Expr<'a>>,
    stmt_arena: &'a Arena<Stmt<'a>>,
) -> Vec<Stmt<'a>>

Remove identity bindings and trivial patterns from residual code.

• Let x = x → removed
• Let x = lit; Return x → Return lit
• Single-arm Inspect with Otherwise only → inline the body
• Iterates to fixpoint (max 4 passes)