Promote local buffers created post fusion to higher memory space

- fusion already includes the necessary analysis to create small/local buffers
  post fusion; allocate these buffers in a higher memory space if the necessary
  pass parameters are provided (threshold size, memory space id)

- although there will be a separate utility at some point to directly detect
  and promote small local buffers to higher memory spaces, doing it while fusion
  when possible is much less expensive, comes free with fusion analysis, and covers
  a key common case.

PiperOrigin-RevId: 232063894