[mypyc] Detect always defined attributes (#12600)
Use static analysis to find attributes that are always defined. Always defined
attributes don't require checks on each access. This makes them faster and
also reduces code size.
Attributes defined in the class body and assigned to in all code paths in
`__init__` are always defined. We need to know all subclasses statically
to determine whether `__init__` always defines an attribute in every case,
including in subclasses.
The analysis looks at `__init__` methods and supports limited inter-procedural
analysis over `super().__init__(...)` calls. Otherwise we rely on intra-procedural
analysis to keep the analysis fast.
As a side effect, `__init__` will now always be called when constructing an object.
This means that `copy.copy` (and others like it) won't be supported for native
classes unless `__init__` can be called without arguments.
`mypyc/analysis/attrdefined.py` has more details about the algorithm in
docstrings.
Performance impact to selected benchmarks (with clang):
- richards +28%
- deltablue +10%
- hexiom +1%
The richards result is probably an outlier.
This will also significantly help with native integers (mypyc/mypyc#837, as tracking
undefined values would otherwise require extra memory use.
Closes mypyc/mypyc#836.
