Better implementation for getItemCount on FlatList (#26164)

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

Libraries/Lists/FlatList.js

@@ -560,7 +560,12 @@ class FlatList<ItemT> extends React.PureComponent<Props<ItemT>, void> {
   };
 
   _getItemCount = (data: ?Array<ItemT>): number => {
-    return data ? Math.ceil(data.length / this.props.numColumns) : 0;
+    if (data) {
+      const {numColumns} = this.props;
+      return numColumns > 1 ? Math.ceil(data.length / numColumns) : data.length;
+    } else {
+      return 0;
+    }
   };
 
   _keyExtractor = (items: ItemT | Array<ItemT>, index: number) => {