qv4: assign split of edges to loop header to the correct group

(i.e assign split edges to the correct loop group, now for real)
The fix of 25b6fae1

 did try fix QTBUG-41766
and simplify the logic to assign the inserted statement to a loop group.
Unfortunately that was incorrect if the target basic block starts a group.
In that case if the source was already inside the group we should add it
to the target basic block, otherwise to the one containing the target
block (as before).
There was no visible regression caused by this.

Change-Id: Id50c42305fc5ac6aedaffa89d8f8dc3b5e976aa4
Task-number: QTBUG-41766
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>