[llvm] [WIP][ADT] Avoid slow size queries on append (PR #136365)

[Nikita Popov via llvm-commits]

nikic wrote:

> @nikic How can I reproduce your compile time benchmarks locally, or find some close enough proxy? Would running opt on a large .bc file under perf be close enough? I think this should be the script used to produce the data: https://github.com/nikic/llvm-compile-time-tracker/blob/master/timeit.sh.

See https://llvm-compile-time-tracker.com/about.php for some information on reproducing. I usually use callgrind for local profiling of small test cases because the results are stable. sqlite3.c is often a good one. Though in this case I'd expect that pretty much any input will work.

https://github.com/llvm/llvm-project/pull/136365