Avoid generating an infinite loop in the case where:
1. Block `first` is adjacent to block `second`, and the branch from `first` to
   `second` is a fallthrough, and
2. Block `second` immediately exits to the interpreter, and
3. Block `second` is invalidated and YJIT is OOM

While pondering how to fix this, I think I've stumbled on another related edge case:
1. Block `incoming_one` and `incoming_two` both branch to block `second`. Block
   `incoming_one` has a fallthrough
2. Block `second` immediately exits to the interpreter (so it starts with its exit)
3. When Block `second` is invalidated, the incoming fallthrough branch from
   `incoming_one` might be rewritten first, which overwrites the start of block
   `second` with a jump to a new branch stub.
4. YJIT runs of out memory
5. The incoming branch from `incoming_two` is then rewritten, but because we're
   OOM we can't generate a new stub, so we use `second`'s exit as the branch
   target. However `second`'s exit was already overwritten with a jump to the
   branch stub for `incoming_one`, so `incoming_two` will end up jumping to
   `incoming_one`'s branch stub.

Fixes [Bug #21257]