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)

This accidentally fixes opt/fehler236.c.

is_Or_Eor_Add(x) is the first test in is_Eor_Add(x).

irgraph: Assert, that set_current_ir_graph() is only used with a graph with IR_GRAPH_CONSTRAINT_CONSTRUCTION.

This case is already handled in equivalent_node_And.

~(~a &  b) =>  a | ~b.
~( a & ~b) => ~a |  b.
~(~a |  b) =>  a & ~b.
~( a | ~b) => ~a &  b.

The initial block is not immature after new_ir_graph() anymore for a long time.
The end block is matured automatically by irg_finalize_cons().

This is unnecessary, because all nodes are created using new_r_*.