Refactor common code getting memref access in getMemRefRegion - NFC

- use getAccessMap() instead of repeating it
- fold getMemRefRegion into MemRefRegion ctor (more natural, avoid heap
  allocation and unique_ptr where possible)

- change extractForInductionVars - MutableArrayRef -> ArrayRef for the
  arguments. Since the method is just returning copies of 'Value *', the client
  can't mutate the pointers themselves; it's fine to mutate the 'Value''s
  themselves, but that doesn't mutate the pointers to those.

- change the way extractForInductionVars returns (see b/123437690)

PiperOrigin-RevId: 232359277