[[unlikely]] ASSERT and minor ASSERT usage changes

[Minty-Meeo]

This PR doubles as a way to ask what are Dolphin Emulator's plans for moving to C++20? Is the issue that GCC still defaults to C++17 in version 12? Other compiler support reasons?

And just how far should [[likely]] and [[unlikely]] be taken? Nullptr checks seem like obvious candidates. Any path that leads to a PanicAlert as well. There are a lot of switch case defaults which only exist to catch stuff which should never happen.

[lioncash]

what are Dolphin Emulator's plans for moving to C++20?

Honestly, I've wanted to move to it for ages now

And just how far should [[likely]] and [[unlikely]] be taken?

Using [[unlikely]] inside of asserts is fine, but for other cases, it's best to have profiling info to back other applications of it (especially for [[likely]], otherwise it makes existing code more noisy (when applied liberally) for (in most cases) negligible benefit

[Minty-Meeo]

Nice, we are finally on C++20!

[shuffle2]

is this mergeable now?

[Pokechu22]

I thought this had actually been merged a while ago. Looks good to me (though it should maybe be rebased to make sure everything still builds properly, and it looks like there's a cassert include in CheatSearchWidget.cpp that's unused (not sure if that already existed).