Auto merge of #138405 - Zoxc:vec-cache-tweaks, r=Mark-Simulacrum

Tweak `VecCache` to improve performance

This has some tweaks to `VecCache` to improve performance.

- It saves a `compare_exchange` in `complete` using the new `put_unique` function.
- It removes bound checks on entries. These are instead checked in the `slot_index_exhaustive` test.
- `initialize_bucket` is outlined and tuned for that.

cc @Mark-Simulacrum

Author: bors