fix fallback impl for select_unpredictable intrinsic

[RedDaedalus]

intrinsics::select_unpredictable does not drop the value that is not selected, but the fallback impl did not consider this behavior. This creates an observable difference between Miri and actual execution, and possibly does not play well with the core::hint version which has extra logic to drop the value that was not selected.

#![feature(core_intrinsics)]

fn main() {
    core::intrinsics::select_unpredictable(true, LoudDrop(0), LoudDrop(1));
    // cargo run: "0"; cargo miri run: "1 0"
}

struct LoudDrop(u8);

impl Drop for LoudDrop {
    fn drop(&mut self) {
        print!("{} ", self.0);
    }
}

This change let me remove the T: [const] Destruct bound as well, since the destructor is no longer relevant.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[rustbot]

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[RalfJung]

Good catch!

Could you add a Miri test?

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[RedDaedalus]

@RalfJung test added!