This fixes opt/fehler222.c.

The newly created nodes have no bitinfo so if we visiting them results
in a crash.

This should fix opt/fehler220.c

This should make the system to find/use alloca more robust.

based on a patch by Luca Barbato <lu_zero@gentoo.org>

This partially fixes opt/fehler217.c.

This fixes C/gnu99/attributes.c with -fdump-all-phases.

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.