[llvm] [ADT] Add implementations for avgFloor and avgCeil to APInt (PR #84431)

[Jay Foad via llvm-commits]

jayfoad wrote:

Incidentally Hacker's Delight has a section "Average of Two Integers" which has some neat tricks for implementing these operations without extending to N+1-bit integers.

https://github.com/llvm/llvm-project/pull/84431