Summary:
Flatlist's `getItemCount` function is frequently called internally by VirtualizedList.
As with other functions, we can remove unnecessary operations with the `numColumns` value.
This makes it much more efficient.
## Changelog
[Internal] [Changed] - Better implementation for getItemCount on FlatList
Pull Request resolved: https://github.com/facebook/react-native/pull/26164
Test Plan: Not required
Differential Revision: D16989335
Pulled By: sahrens
fbshipit-source-id: b0075b2c2aeb9b9d7644c8bb18702a7cca8a4dce