[llvm] [SmallPtrSet] Optimize Grow and inline FindBucketFor. NFC (PR #206304)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke approved this pull request.

LGTM. Although it looks like this is a small improvement only, as (a) grow doesn't happen often and (b) most probably the first inspected bucket is empty, so no win for the common case.

https://github.com/llvm/llvm-project/pull/206304