Auto merge of #150386 - saethlin:no-const-assert-likely, r=tgross35

Remove the explicit branch hint from const_panic

This was asked for in PR review for equivalence with `assert!`, but we don't need likely/unlikely intrinsics here (and indeed they are a bit of an antipattern), because codegen knows about the pattern that `assert!` produces where only one target of a SwitchInt leads to a block terminated by a `#[cold]` call. All our panic entrypoints are `#[cold]`.

`assert!` does not use the likely/unlikely intrinsics, maybe it did in the past.

So the intrinsic call in `const_assert!` should be completely unnecessary. But currently it isn't, because `const_panic!` stamps out a runtime wrapper for its entrypoint which is not given `#[inline]`, citing compile times. It's not obvious to me why not using `#[inline]` would even be good in this case.

I think the runtime wrapper should have `#[inline]` or `#[cold]`, so I'm going to do 3 perf experiments here.
Just changing `const_assert!`: https://perf.rust-lang.org/compare.html?start=fabece9e9491d0a3c3655dba488881968b7c5f7a&end=f6b082cfcf880ad80b33523d5047c11654d907a8&stat=instructions:u
Also adding `#[inline]` to `const_panic!`'s runtime wrapper: https://perf.rust-lang.org/compare.html?start=fabece9e9491d0a3c3655dba488881968b7c5f7a&end=f467cc7e49b74b47da0e2cde49fc1a3140e00ff6&stat=instructions:u
Also adding `#[cold]` to `const_panic!`'s runtime wrapper: https://perf.rust-lang.org/compare.html?start=2e854a9344154564259de51385e9ec9506c0f3b7&end=fd8e098558ff08ac002b05ff78e9f91ffa6f3738&stat=instructions:u

The previous comment indicated perf would be worse with `#[inline]` on the runtime branch, but the results in this PR show that it is more mixed, and net positive. And in fact, the only regression in serde seems to be because we created a very tiny additional amount of codegen, but pushed the size of the second CGU just high enough that we no longer merge all CGUs together. So now serde (in debug with incremental disabled) has 2 CGUs instead of 1. If we just so happened to be still below the threshold where the CGUs don't merge, adding `#[inline]` would have been nearly all improvements.