The previous approach put the irg link into the block attributes, so
while saving memory an extra indirection was necessary. Some testing
with valgrind massif+callgrind revealed that a direct link increses peak
memory usage by ~1% while also increasing runtime by ~1%. More
importantly the code gets a bit simpler and some strange corner cases can be
removed with the direct link.

Make the list scheduler a set of helper functions, instead of a complex
design with a set of callbacks.
Simplified list scheduler code.

In their current state they produce very bad code and are just a
maintenance burden.

This fixes backend/sparc_segfault.c.

This fixes a subtle bug where the delay slot filler would move a
spill/reload across a MemPerm although they both share a spillslot.

The global analysis has quadratic worst-case complexity in the number of
graphs.  At least if alias analysis is disabled anyway, this can be
avoided.

This avoids strangely large positive numbers instead of negative offsets
on x86_64 hosts.

Privously nodes that only had a keep-alive users would be moved up but
never down again.

The problem was that we have to force SSA construction for memory values to
ensure the presence of the Phi[loop] nodes. Previously we did not do it
if we didn't introduce new values.

This fixes firm-testsuite/opt/jumpthreading8.c

This should really fix opt/fehler218.c oddities.