[llvm] [Transforms] Introduce BuildBuiltins.h atomic helpers (PR #134455)

[Nikita Popov via llvm-commits]

nikic wrote:

> @jyknight Here the proof that emitting sized libcall atomics early is better than letting AtomicExpandPass do it: https://godbolt.org/z/bGW37TMYh. In this case it skips tail call elimination.

This looks like it should be simple enough to fix in AtomicExpand. We could export markTails() from TRE and run it if AtomicExpand emitted any calls.

https://github.com/llvm/llvm-project/pull/134455