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This fixes opt/fehler206.c.

before we only tested, if we produce an output that is used as as input
of any schedule node in between. This missed the case where you have
unscheduled utility nodes like Sync in between.

The interface was really inefficient and shouldn't be used like that.
So simply don't provide functions that make inefficient code convenient.

We missed the case where transform_node created a node that
computed_value or equivalent_node would catch

It was a strange feature only inspired by the lazyness of some frontend
developer (=me)

they were in lower_mode_b but just generically apply to all Mux(c, 0, 1)

Some peephole optimisations enable others, so splitting into multiple
passes makes sense.

A test can exactly be removed if we are only interested in the sign and
zero flag. Rewrote and refactored code.
Now we don't miss some cases anymore where only sign was requested.

Add op(op(x, c0), c1) => op(x, op(c0, c1)) as localopt rule. This is an
very usual and often encountered case so it makes sense to have this
working without explicitely invoking the associativity optimisation phase.

when bits are flipped for a bittest we can just as well negate the pnc
and don't flip the bits

The preconditions were too tight

This is the new way of handling switch-jumps. The node contains a table
which maps (ranges of) input values to proj numbers. Compared to a
Cond-node this results in a clean consecutive sequence of Proj numbers
(no searching for a free number for the default_pn anymore) and allows
factoring multiple cases jumping to the same block in a single Proj
(though we still need the optimisation in cfopt for that).

They are reserved in ANSI-C so instead of _XXX we use XXX_ now.

(they were introduced when moving value_classes out of the descriptor)