[CodeGenSchedule] Don't allow invalid ReadAdvances to be formed (#82685)

Forming a `ReadAdvance` with an entry in the `ValidWrites` list that is
not used by any instruction results in the entire `ReadAdvance` to be
ignored by the scheduler due to an invalid entry.

The `SchedRW` collection code only picks up `SchedWrites` that are
reachable from `Instructions`, `InstRW`, `ItinRW` and `SchedAlias`,
leaving the unreachable ones with an invalid entry (0) in
`SubtargetEmitter::GenSchedClassTables` when going through the list of
`ReadAdvances`