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When a node is rematerialized because its flag result is needed, rewire
later uses of this node (but not the flags) to use the rematerialized
node's results. This reduces the lifetime of the node's results.

Now that the flag outputs of arithmetic operations are used, a node may
have multiple connections to the flag producer, but only one of these
actually reads the flags.

This fixes opt/fehler265.c.

This fixes opt/fehler263.c.

This makes our usage of PhiMs to represent the observable behaviour of
non-terminating loops more obvious.
Having an explicit loop Phi constructor also fixes problems with Phi
getting optimized before the keep edge was added.

Settle on 1 flag type and no different I_FLAG and E_FLAG

The global switch is not necessary as the specific switches can express
the same options.

The reassoc running flag wasn't necessary in new reassoc versions
anyway.

This fixes opt/fehler262.c.

This should fix the case where it would remove an unused PhiM in an endless
loop (which is not correct).

A self-loop before the first real value was not recognized before and
could lead to an invalid kept-PhiM removal.

This fixes opt/fehler261.c.

For details see endless loops section on the homepage.

The semantics on when to set this flag were unclear, so there weren't
any real users anyway.