Rework load memory chain walking:
- We keep the pointer as a base + offset representation all the time
- Reuse predict_load code from localopt for more accurate replacement of
  Loads from constant entities
- Support some more cases where load-after-load and load-after-store did
  not use the same mode but where we can easily get the value with some
  arithmetic.

This fixes opt/loadstore6.c.

This fixes opt/fehler238.c.

This uses a more pleasing very light blue background for blocks and adds
some comments on how the colors were defined with the hsv color model.

- no need for Pin special case
- use color_anchor for End node even though it has mode_X

It's safe to pass a null pointer to set_Block_entity().

This fixes gcc/pr36988.c

- Does not invalidate graph properties anymore if nothing changed
- Only produce Bad/Jmp Tuple predecessors for possible throwing
  Load/Store nodes
- Improve check_load_store() logic slightly and rename to
  is_conv_bitcast()

try to protect users from adding spills/stores to the frame entity coalescer (only loads may be added)