`is_char_boundary` not elided when impl `pop` in terms of `truncate`

[Rudxain]

I've implemented String::pop by using String::truncate internally. The UTF-8 boundary assertion check is still included in the release binary (for x86 targets). I can reproduce the lack of optimization on both 1.88.0-nightly (2025-04-07 e643f59) and stable (1.86.0)

Playground

I haven't tested if both functions are semantically-identical, but I've read the source-code (and docs) of both pop and truncate, so I am confident that pop_trunc is correct

@rustbot label: +I-slow