QComboBox::showPopup() should use `usePopup` consistently

`QComboBox::showPopup()` caches the widget's SH_ComboBox_Popup style hint very early, but does not use the cached value throughout. I don't think that value can change (except for styles that have randomisation feature?), so there shouldn't be anything against using it in all places where the style hint is checked.

I discovered this as a result of a crash executing the `view()->scrollTo()` instruction where for some reason the cached `style` pointer variable got corrupted (or the style somehow changed during the `showPopup()` call?!; either way undoubtedly a bug that's not in but only messes with Qt code).