Check number of cells referenced by group instead of group size

It's more relevant when deciding whether to use OpenCL or not.

Note that we won't use OpenCL for a single formula cell, no matter how large a
calculation it invokes (like =SUM(A1:A1000000), for instance), as a single
cell is not a group.

Change-Id: I66b03c197431c2b4cef96f46b010d99d3e0624fc