Assure that there are no tuples when calculating post-dominators.
Assure that there are no tuples when calculating post-dominators.

The dominator calculation assures the same.
This fixes the optimize build for several test cases.

libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,MatzeB/libfirm,libfirm/libfirm