[SLP] Remove ScheduleData::UnscheduledDepsInBundle field [NFC-ish]

We can simply compute the value of this field on demand.  Doing so clarifies the behavior when one of the instructions within a bundle doesn't have valid dependencies.  I vaguely thing this could change behavior slightly, but none of the test cases are affected, and my attempts to write one by hand have failed.

This also minorly reduces memory usage, but that's a secondary value at best.