This fixes backend/shift.c.

This fixes opt/ifconv6.c.

The local optimization should now match the prediction of
ir_is_optimizable_mux. This fixes opt/mux.c on SPARC.

They are located in the start block, which dominates all other blocks.

Dominance is reflexive.

This way we can express that the algos work on 32bit numbers only
instead of asserting that the unsigned type is 32bits wide.

This fixes backend/minus64.c.

This should fix C/bitfield4.c on leon.