Auto merge of #155550 - zetanumbers:cache_insert_unique, r=oli-obk

Replace `ShardedHashMap` method `insert` with debug-checked `insert_unique`

Currently every use of `ShardedHashMap::insert` checks that it won't evict an old value due to unique key. I haven't found any issue related to that faulty condition, so I thought of replacing it with `ShardedHashMap::insert_unique` which doesn't check for this condition unless `debug_assertions` are enabled. This might improve the performance.

r? @petrochenkov

Author: bors