fix(ssa refactor): safe to query cfg for single block programs

[joss-aztec]

Description

Problem*

Loop unrolling was crashing on single block programs due the cfg not holding a node for the entry block (which had no edges)

Message:  ICE: Attempted to iterate predecessors of block not found within cfg.
Location: crates/noirc_evaluator/src/ssa_refactor/ir/cfg.rs:102
...
  18: noirc_evaluator::ssa_refactor::ir::cfg::ControlFlowGraph::predecessors::h40675d9252c86030
      at /Users/aztec/Documents/GitHub/noir-lang/noir/crates/noirc_evaluator/src/ssa_refactor/ir/cfg.rs:100
  19: noirc_evaluator::ssa_refactor::opt::unrolling::find_all_loops::h750f7171170cccd7

Summary*

Init the cfg with an empty node for the entry block

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

Thank you!