Add rationale for `rustc_type_ir` behind allowing rustc::potential-query-instability lint

Context: https://github.com/rust-lang/rust/issues/84447

Motivation:

1. This lint asks to do so. An example:

   error: using `values` can result in unstable query results
      --> compiler/rustc_query_system/src/dep_graph/serialized.rs:654:50
       |
   654 |             let mut stats: Vec<_> = record_stats.values().collect();
       |                                                  ^^^^^^
       |
       = note: if you believe this case to be fine, allow this lint and add a comment explaining your rationale
   

2. This would prevent further fix attempts so wastes of time as well.
3. Nearly all of those allowances has a comment which explains the rationale.

To do: Add a comment explaning the rationale behind allowing rustc::potential-query-instability lint for all occurrences in the file compiler/rustc_type_ir/src/search_graph/mod.rs. You can find the places where this lint is triggered by searching with #[allow(rustc::potential_query_instability)] in this file.

Open compiler/rustc_type_ir/src/search_graph/mod.rs and search for #[allow(rustc::potential_query_instability)] to locate every occurrence. Read the surrounding code and existing comments to understand each allowance, then add a rationale comment for each one. Done means all occurrences in the file explain why the allowance is intentional.