Auto merge of #142095 - joshtriplett:optimize-veccache, r=SparrowLii

Simplify and optimize `VecCache`'s `SlotIndex::from_index`

Simplify and optimize `SlotIndex::from_index`

Break out bucket 0 (containing `idx < 4096`) as an early return, which
simplifies the remainder of the function, and allows optimizing the
`checked_ilog2` since it can no longer return `None`.

This reduces the runtime of `vec_cache::tests::slot_index_exhaustive`
(which calls `SlotIndex::from_index` for every `u32`, twice) from ~15.5s
to ~13.3s.

Separately, simplify the test case as well. (The old and new code passes with
the old and new test case.)

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Noticed because `slot_index_exhaustive` stood out as taking unusually long compared to other tests, so I started investigating what it was doing.

Author: bors