There are four versions of a conditional branch: to be predicted, almost always taken, almost never taken, and too random to waste a predictor on. Compilers nowadays have "likely" and "unlikely" intrinsics, but offer, yet, none for the last. I think x86 now ignores its "likelihood" branch prefix because back when introduced it was too often wrong; and now compilers don't emit it because it they know it is ignored.

The "too random to predict" would be good for sorting, where it could provide a 2x speedup. To get the effect today you need to use conditional-move instructions, which have become unfashionable. Getting your compiler to emit a cmov is tricky.

Intel added a special instruction (maybe in skylake?) to use for spinning on an atomic flag update, that just halts until the cache line being watched gets clobbered by a message from some other cache. Compilers don't emit it, unfortunately, even when spinning on what they know is an atomic flag.

There are zillions of such instructions that were good ideas but were never taken up by programmers who they were meant for.