Fix compiler warnings with mingw64.

Task-number: QTBUG-34152

Change-Id: Ibb93d1cac8c343a7ca34ce7d010f24fc56ba89df
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
V4 SSA: speed up dominator calculations.

Changed three recursive routines to worklist-based iterative ones. This
not only speeds up the dominator frontier calculation, but also prevents
the algorithm to run out of stack space.

This is a partial fix for QTBUG-34047.

Change-Id: Ife8dc35724d50408ad356e1621884bdb82db9626
Reviewed-by: Lars Knoll <lars.knoll@digia.com>