This way it can be tested and used as an optimization in some places.

This fixes backend/sparc_finish.c.

This fixes backend/peephole.c.

This fixes backend/chordal.c.

This fixes opt/ifconv9.c.

You never know if the host long can hold enough values for the mode of
the target machine.

This fixes opt/dwlower2.c.

- x & 3  => x % SC_BITS
- x >> 2 => x / SC_BITS
(after everything is converted to unsigned this should result in the
 exact same code as before).