Add parsing to file check (#18304)
Summary:
This allows you to embed checks in IR, making the test more readable.
E.g.
```
graph_str = 'graph(%0 : Double(5, 5)):
# CHECK: aten::relu
%1 : Double(5, 5) = aten::relu(%0)
return (%1)'
FileCheck().run(graph_str, parseIR(graph_str))
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18304
Differential Revision: D14652372
Pulled By: eellison
fbshipit-source-id: 7430b9d1dc2b7584704375aac02d7392ecec76a0
