[SlotIndexes] Use upper/lower bound terminology for MBB searches. NFC. (#68802)

Rename advanceMBBIndex and findMBBIndex to getMBBLowerBound and add
getMBBUpperBound.

The motivations are:
- Make it clear what kind of search is being done, using names inspired
  by std::upper/lower_bound.
- Simplify getMBBFromIndex which really wants an upper bound search and
  previously had to work hard to get the result it wanted from a lower
  bound search.