mypy crash when variable defined after the function

[yoav-orca]

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?
  A bug
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

import re

def match(s: str) -> None:
    if m := rx.match(s):
        print(m)

rx = re.compile("X")

• What is the actual behavior/output?

mypy --strict "scratch_4.py" --show-traceback
scratch_4.py:4: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.rtfd.io/en/latest/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.782
Traceback (most recent call last):
  File "mypy/checker.py", line 401, in accept
  File "mypy/nodes.py", line 1195, in accept
  File "mypy/checker.py", line 3212, in visit_if_stmt
  File "mypy/checker.py", line 3928, in find_isinstance_check
  File "mypy/checker.py", line 4104, in find_isinstance_check_helper
  File "mypy/checker.py", line 4108, in find_isinstance_check_helper
KeyError: <mypy.nodes.NameExpr object at 0x110664120>
scratch_4.py:4: : note: use --pdb to drop into pdb

• What is the behavior/output you expect?
  To type check successfully

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?
  I'm using CPython 3.8.4 & mypy 0.782, it reproduces with mypy 0.790+dev.a5455bd9f37825d24415e46a4d278d71502c9d07

• What are the mypy flags you are using? (For example --strict-optional)
  --strict

• If mypy crashed with a traceback, please paste
  the full traceback below.

Traceback (most recent call last):
  File "mypy/checker.py", line 401, in accept
  File "mypy/nodes.py", line 1195, in accept
  File "mypy/checker.py", line 3212, in visit_if_stmt
  File "mypy/checker.py", line 3928, in find_isinstance_check
  File "mypy/checker.py", line 4104, in find_isinstance_check_helper
  File "mypy/checker.py", line 4108, in find_isinstance_check_helper
KeyError: <mypy.nodes.NameExpr object at 0x110664120>

(You can freely edit this text, please remove all the lines
you believe are unnecessary.)

[ilevkivskyi]

I think this is a duplicate of #9054