Skip debug dep node code in non-incremental mode

[zetanumbers]

Issues happen when trying to use incremental_info with compiler's debug_assertions enabled to access more info, like the "Total Duplicate Edge Reads" counter.

[rustbot]

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[lcnr]

I lack a lot of context to understand these changes. Can you go into more detail for how things are going wrong/what this is fixed

[alex-semenyuk]

@zetanumbers
Thanks for your contribution
From wg-triage. Could you please handle suggestion from comment above

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[zetanumbers]

So if you use -Zincremental-info flag compiler will print out DepGraph statistics like this:

[incremental]
[incremental] DepGraph Statistics
[incremental] ------------------------------------------------------------------------------------------
[incremental]
[incremental] Total Node Count: 373217
[incremental] Total Edge Count: 2793009
[incremental]
[incremental]  Node Kind                           | Node Frequency   | Node Count  | Avg. Edge Count  |
[incremental] ------------------------------------------------------------------------------------------
[incremental]  def_kind                            |             3.3% |       12249 |              1.0 |
[incremental]  def_span                            |             3.0% |       11339 |              2.0 |
[incremental]  def_ident_span                      |             2.9% |       10906 |              2.0 |
[incremental]  local_def_id_to_hir_id              |             2.9% |       10699 |              1.0 |
[incremental]  evaluate_obligation                 |             2.8% |       10579 |              3.1 |
[incremental]  generics_of                         |             2.7% |       10125 |              4.0 |
...
[incremental]  entry_fn                            |             0.0% |           1 |              0.0 |
[incremental]  crate_inherent_impls_validity_check |             0.0% |           1 |              1.0 |
[incremental] ------------------------------------------------------------------------------------------
[incremental]

But if you build rustc with rust.debug = true in the bootstrap.toml, which enables cfg(debug_assertions) in rustc code, then it would also include:

[incremental] Total Edge Reads: 10273855
[incremental] Total Duplicate Edge Reads: 7101859

[zetanumbers]

However now master behaves correctly so I'm closing this PR.