Removing all unsafe casts

- some of them are there to provide more definite type info
- and most of them to indicate the guarantee of a value being not null
There are removed because they should not be necessary, and will probably be removed from Graal entirely eventually

Signed-off-by: Stefan Marr <git@stefan-marr.de>

Conflicts:
	src/som/interpreter/nodes/ContextualNode.java
Remove purely local variable access nodes.

Signed-off-by: Stefan Marr <git@stefan-marr.de>
The receiver should always be read from the arguments array

Signed-off-by: Stefan Marr <git@stefan-marr.de>
Fix splitting of NonLocalVariableNodes

Signed-off-by: Stefan Marr <git@stefan-marr.de>