[mypyc] Try adjusting error kinds for all blocks (#15262)

`adjust_error_kinds()` should be run over every block, but the old impl.
of `insert_exception_handling()` would stop iterating over blocks once
it generated a default error handler. This made sense for its original
purpose, but unfortunately limits the effectiveness of
`adjust_error_kinds()`.

Changing `insert_exception_handling()` to loop over every block no
matter what also means we can get rid of the weird GetAttr special
casing in the branch emit logic (stumbling across that is what made me
investigate in the first place!).

This reduces self-compile LOC by 1.1% in my experiments (avoiding error
branches sometimes eliminates register assignments or on-error
decrefs... according to my brief look through the C diff between master
and this patch).

| Revision | Self-compile C LOC |
|--------|--------|
| PR | 2 265 486 (**-1.1%**) | 
| Master (905c2cb) | 2 290 748 |

Notable removals in the diff (other than the all of the label
renumbering) ...[^1]

![image](https://github.com/python/mypy/assets/63936253/3efee503-0c63-441f-9ce8-2e4d7083d5e5)

![image](https://github.com/python/mypy/assets/63936253/06136e2a-4315-40aa-9ca5-ec72bbd7ac45)

[^1]: apologies for the screenshots, this diff was generated during a
SSH session so copying and pasting is a real pain.

Author: ichard26

mypyc/codegen/emitfunc.py

@@ -197,13 +197,6 @@ def visit_goto(self, op: Goto) -> None:
 
     def visit_branch(self, op: Branch) -> None:
         true, false = op.true, op.false
-        if op.op == Branch.IS_ERROR and isinstance(op.value, GetAttr) and not op.negated:
-            op2 = op.value
-            if op2.class_type.class_ir.is_always_defined(op2.attr):
-                # Getting an always defined attribute never fails, so the branch can be omitted.
-                if false is not self.next_block:
-                    self.emit_line(f"goto {self.label(false)};")
-                return
         negated = op.negated
         negated_rare = False
         if true is self.next_block and op.traceback_entry is None:

mypyc/transform/exceptions.py

@@ -43,18 +43,16 @@ def insert_exception_handling(ir: FuncIR) -> None:
     # Generate error block if any ops may raise an exception. If an op
     # fails without its own error handler, we'll branch to this
     # block. The block just returns an error value.
-    error_label = None
+    error_label: BasicBlock | None = None
     for block in ir.blocks:
         adjust_error_kinds(block)
-        can_raise = any(op.can_raise() for op in block.ops)
-        if can_raise:
-            error_label = add_handler_block(ir)
-            break
+        if error_label is None and any(op.can_raise() for op in block.ops):
+            error_label = add_default_handler_block(ir)
     if error_label:
         ir.blocks = split_blocks_at_errors(ir.blocks, error_label, ir.traceback_name)
 
 
-def add_handler_block(ir: FuncIR) -> BasicBlock:
+def add_default_handler_block(ir: FuncIR) -> BasicBlock:
     block = BasicBlock()
     ir.blocks.append(block)
     op = LoadErrorValue(ir.ret_type)