Reassoc: Construct constant 0 if all nodes cancel out.
Reassoc: Construct constant 0 if all nodes cancel out.

This fixes opt/reassoc040.c

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