Get away from the strange set_default_XXX_op style and instead let the
code modules register their node ops directly.

The opcode list was a member of irprog before which wasn't really
handled consistently. Also make sure opcodes are properly freed at
ir_finish().

memops are nodes that have memory inputs, you can generically query them
for their memory input. We can also get rid of get_fragile_op_mem in
favor of get_memop_mem.

Some backends create nodes in their lower_for_target phases already.
These nodes are placeholders for real backend nodes later, in contrast
they do not have register requirements or backend attributes. Simplified
this by allowing custom dumpers for them and leaving out the backend
node structs.

- The pns of X_regular and X_except are annotated in the opcode now.
- The memory input is annotated in the opcode now
- only nodes with X_regular, X_except are marked fragile

add unknown_jump opflag for the special case of a jump where we can't directly influence the destination and can't predict/change them

[r28143]

[r28127]

[r28038]

[r28015]

[r28010]

[r27924]

[r27846]

[r27836]

[r27828]

[r27827]

[r27788]

[r27762]

change backend node specification to use whole words instead of cryptical 1-char abreviations for flags

[r27745]