Rework exception compiler by youknowone · Pull Request #6909 · RustPython/RustPython

Move exception handler tracking from compile-time fblock metadata to
a post-codegen CFG analysis pass, matching flowgraph.c's pipeline.

Compiler changes (compile.rs):
- Remove fb_handler, fb_stack_depth, fb_preserve_lasti from FBlockInfo
- Remove handler_stack_depth() and current_except_handler() helpers
- Emit SetupFinally/SetupCleanup/SetupWith/PopBlock pseudo instructions
  instead of manually tracking handlers in fblocks
- Add dead blocks after raise/break/continue/return to prevent
  dead code from corrupting the except stack
- Add missing PopTop after CallIntrinsic1 ImportStar
- Fix async comprehension SetupFinally/GetANext emission order
- Fix try-except* handler: move BUILD_LIST/COPY before handler loop
- Rework unwind_fblock for HandlerCleanup, TryExcept, FinallyTry,
  FinallyEnd, and With/AsyncWith to emit proper PopBlock sequences

New CFG analysis passes (ir.rs):
- mark_except_handlers(): mark blocks targeted by SETUP_* instructions
- label_exception_targets(): walk CFG with except stack to set
  per-instruction handler info and convert POP_BLOCK to NOP
- convert_pseudo_ops(): lower remaining pseudo ops after analysis
- Compute handler entry depth from SETUP_* type in max_stackdepth()
  (SETUP_CLEANUP=+2, SETUP_FINALLY/SETUP_WITH=+1)
- Fix SEND jump_effect from -1 to 0 (receiver stays until END_SEND)
- Add underflow guard for handler stack_depth calculation

Instruction metadata (instruction.rs):
- SetupCleanup/SetupFinally/SetupWith now carry target: Arg<Label>
- Add is_block_push()/is_pop_block() to PseudoInstruction/AnyInstruction
- Fix LoadSpecial stack_effect from (2,2) to (1,1)
- Set Setup* stack_effect_info to (0,0) for fall-through consistency

Fixes 3 test_coroutines expectedFailures:
- test_with_8, test_for_assign_raising_stop_async_iteration{,_2}