Check for equal modes before subtracting them.
Check for equal modes before subtracting them.

This fixes opt/boolopt4.c.

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