Factor out code to test whether a node is a Const equal to a mode size minus 1.
Factor out code to test whether a node is a Const equal to a mode size minus 1.

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