Rollup merge of #71688 - ecstatic-morse:const-downcast, r=oli-obk

Rollup merge of #71688 - ecstatic-morse:const-downcast, r=oli-obk

Allow `Downcast` projections unconditionally in const-checking

`ProjectionElem::Downcast` sounds scary, but it's really just the projection we use to access a particular enum variant. They usually appear in the lowering of a `match` statement, so they have been associated with control flow in const-checking, but they don't do any control flow by themselves. We already have a HIR pass that looks for `if` and `match` (even ones that have 1 or fewer reachable branches). That pass is double-checked by a MIR pass that looks for `SwitchInt`s and `FakeRead`s for match scrutinees. In my opinion, there's no need to look for `Downcast` as well.

r? @oli-obk