arm: Just use be_upper_bits_clean().
arm: Just use be_upper_bits_clean().

This avoids code duplication and actually works correctly.
This fixes opt/convs.c.

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