Auto merge of #141039 - lqd:expensive-sanity, r=compiler-errors

move expensive layout sanity check to debug assertions

It is [hard to fix](https://github.com/rust-lang/rust/issues/141006#issuecomment-2883415000) the slowness in the uninhabitedness computation for very big types but we can fix the very specific case of them being called during the layout sanity checks, as described in #140944.

So this PR moves this uninhabitedness check to the other expensive layout sanity checks that are ran under `debug_assertions`.

It makes building the `lemmy_api_routes` crate's self-profile `layout_of` query go from

```
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
| Item                                                   | Self time | % of total time | Time     | Item count | Incremental result hashing time |
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
| layout_of                                              | 63.02s    | 41.895          | 244.26s  | 123703     | 50.30ms                         |
+--------------------------------------------------------+-----------+-----------------+----------+------------+---------------------------------+
```

on master (2m17s total), to

```
| layout_of                                              | 330.21ms  | 0.372           | 26.90s   | 123703     | 53.19ms                         |
```

with this PR (1m15s total).

(Note that the [perf run results](https://github.com/rust-lang/rust/pull/141039#issuecomment-2884688756) below look a bit better than [an earlier run](https://perf.rust-lang.org/compare.html?start=4eca99a18eab3d4e28ed1ce3ee620d442955a470&end=c4a00993f8ee02c7565e7be652608817ea2fb97d&stat=instructions:u) I did in another PR. There may be some positive noise there, or post-merge results could differ a bit)

Since we discussed this today, r? `@compiler-errors` — and cc `@lcnr` and `@RalfJung.`

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