Rollup merge of #123089 - Philippe-Cholet:vecdeque_pop_assume_cap, r=Nilstrieb

Add invariant to VecDeque::pop_* that len < cap if pop successful

Similar to #114370 for VecDeque instead of Vec.

I initially come from rust-itertools/itertools#899 where we noticed that `pop_front;push_back;` was slower than expected so `@scottmcm` suggested I file an issue which lead to https://internals.rust-lang.org/t/vecdeque-pop-front-push-back/20483 where **kornel** mentionned #114334 (fixed by #114370).

This is my first time with codegen tests, I based the test on what was done for Vec.

Author: matthiaskrgr