Auto merge of #17639 - Veykril:salsa-perf, r=Veykril

Some more small salsa memory improvements

This does limit our lru limits to 2^16 but if you want to set them higher than that you might as well not set them at all. Also makes `LRU` opt-in per query now, allowing us to drop all the unnecessary LRU stuff for most queries

Author: bors