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

[Nikita Popov via llvm-commits]

nikic wrote:

> @nikic Would you be able to see if this helps with compile times?

The change itself has a very small negative effect: https://llvm-compile-time-tracker.com/compare.php?from=f696508b38d1958aac5992a98a90fe6e773e8709&to=a69d10e1ebd2e99155aa893ee9b337bf7b3aadd3&stat=instructions:u

If I restore the append_range use in BFI I still get a regression: https://llvm-compile-time-tracker.com/compare.php?from=a69d10e1ebd2e99155aa893ee9b337bf7b3aadd3&to=2e17e3511a77e2f054c9cb215d3156f497c39ac0&stat=instructions:u

It would not have expected the regression to still be there with this change, so I suspect something isn't right...

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