+/**
+ * Handle register targeting constraints signaled by a Perm.
+ * @param alloc_env Private data for the allocation phase.
+ * @param perm The Perm node guarding the constrained node.
+ * @return The constrained node.
+
+ Pro-coloring works as follows:
+
+ +-----------------------------------+
+ | Perm |
+ +---.-------.--------.---------.----+
+ | | | |
+ +---+--+ | | |
+ | Proj | | | |
+ +------+ | | |
+ | | |
+ +--+---+ | |
+ | Proj | | |
+ +--.---+ | |
+ | | |
+ | +--+---+ |
+ | | Proj | |
+ | +------+ |
+ | |
+ | +---+--+
+ `-. | Proj | Result:
+ `._ +---.--+ R1
+ `. |
+ `-. |
+ `._ |
+ +`.-+--+
+ |Constr| Result:
+ +------+ R2
+
+ 1) Look at all Projs of the Perm if they have output constraints.
+ If one has an output constraint, pre-color it, else record it
+ in the set leftover. Its color has to be chosen after all
+ constrained nodes are colored. Furthermore record all colors
+ used in the pre-coloring in the set colors_used.
+
+ 2) Look whether the first node not a Proj (this is the constrained
+ node due to which the Perm has been inserted) has an output
+ constraint. If yes, pre-color the node accordingly else do nothing
+ since the node's input constraints are modelled by the Proj's
+ output constraint.
+
+ There's one subtle point here: If thenode has an output constraint
+ and the live range of some Proj ends at that node, we must give
+ that Proj the color of the constrained node. Otherwise the
+ available colors may not suffice for the rest of the projs.
+
+ 3) At last, color the Projs which have not been colored yet with the
+ left over colors.
+
+ So afterwards, everything including the constrained node will
+ be colored and the assign() phase can complete this coloring.
+ Note that therefore, we put the pre-colored nodes in a set
+ called pre_colored().
+
+ */
+static ir_node *handle_constraints_at_perm(be_chordal_alloc_env_t *alloc_env, ir_node *perm)