metacompile isinstance checks (#23885)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23885
This is a series of PRs that will allow us to support adding [padding to conv](https://github.com/pytorch/pytorch/pull/22484) and also reduce the friction of adding method overloads that was brought up in https://github.com/pytorch/pytorch/pull/23266.
This PR only compiles one if branch if the condition is an isinstance check. This is consistent with what mypy does; it does not report errors if a branch can be determined statically to be unreachable.
```
def foo(x):
# type: (int) -> int
if isinstance(x, str):
return x["1"]
return x + 1
reveal_type(foo) # no error, shows int -> int
```
Test Plan: Imported from OSS
Differential Revision: D16697092
Pulled By: eellison
fbshipit-source-id: d3eb4925cd16d551515ac6ff620a69897dbec130
