add support for breaks and continues (#21692)
Summary:
Add support for breaks and continues in the jit. We do with a Graph transform pre-SSA.
A graph of the form
```
def test():
while i < 5:
if i == 3:
break
i += 1
print(i)
```
has the body of the loop transformed to
```
if i == 3:
did_break = True
else:
did_break = False
if did_break:
loop_exit = True
else:
i += 1
print(i)
loop_exit = i < 5
```
I am going to add more tests but I think it is ready for review now.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21692
Differential Revision: D16215807
Pulled By: eellison
fbshipit-source-id: 365102f42de4861d9323caaeb39a96de7619a667
