flush keepalives on operator assignment statements (python#13144)

This PR makes mypyc flush keepalives on operator assignment statements, to prevent accessing undefined variables in the generated C code.

Fixes mypyc/mypyc#941.

mypyc/irbuild/statement.py

@@ -136,6 +136,7 @@ def transform_operator_assignment_stmt(builder: IRBuilder, stmt: OperatorAssignm
     # usually operator assignments are done in-place
     # but when target doesn't support that we need to manually assign
     builder.assign(target, res, res.line)
+    builder.flush_keep_alives()
 
 
 def transform_import(builder: IRBuilder, node: Import) -> None:

mypyc/test-data/run-generators.test

@@ -595,3 +595,15 @@ class C:
         self.foo.flag = True
         yield
         self.foo.flag = False
+
+[case testGeneratorEarlyReturnWithBorrows]
+from typing import Iterator
+class Bar:
+    bar = 0
+class Foo:
+    bar = Bar()
+    def f(self) -> Iterator[int]:
+        if self:
+            self.bar.bar += 1
+            return
+        yield 0