as i already say, this member is used only in this block of code. it has sense only for visible controls. something more, in old code it can cause erroneous added dirty region of invisible control to global list of dirty regions. that is its only purpose, to add region to global list.
when control become visible, its region will be any way added to global list just because of return value of CGUIControl::Animate few lines above, so value of m_controlIsDirty is useless in that transition but still reset at end of the block.

but if you still want that, i can put this outside of block. one ( not needed ) boolean assignment will not hurt at all. for now i prefer to keep it inside.