[llvm] [ADT] Add roundUpNumBuckets to DenseMap (NFC) (PR #168301)

[Alexis Engelke via llvm-commits]

aengelke wrote:

+0.35% regression on instructions:u when building with GCC: http://llvm-compile-time-tracker.com/compare.php?from=6152a8b31003d38eefb5dd9023a6c7e4e4e09ac4&to=4206558f715b7a67daa9c39528d04106f88612b4&stat=instructions:u

https://github.com/llvm/llvm-project/pull/168301