Optimize away multiple addpattern definitions in sequence

We should translate
```python
def func($1):
$2
@addpattern(func)
def func($3):
$4
```
into
```python
def func(*args, **kwargs):
kwargs_ = kwargs.copy()
match_args_kwargs $1:
$2
return None
kwargs = kwargs_
match_args_kwargs $3:
$4
return None
```