Move the code for "fill simple" to its own method.

Change-Id: I317f20eb4e8b6e41f12b7da872d158b859579861
Speed up simple fill of formula cells. Vertical direction only.

Other cells could be optimized in a similar fashion, but they aren't
that slow even with the current code.

Change-Id: I0b71bf271fab68c9fba0b39463dfe3ef46dacfb0