Correctly process nested definitions in astmerge (#14104)

Fixes #12744

The fix is straightforward. Current logic can produce a random mix of
old and new nodes if there are functions/methods nested in other
statements.

Author: ilevkivskyi

mypy/server/astmerge.py

@@ -213,8 +213,8 @@ def visit_mypy_file(self, node: MypyFile) -> None:
         super().visit_mypy_file(node)
 
     def visit_block(self, node: Block) -> None:
-        super().visit_block(node)
         node.body = self.replace_statements(node.body)
+        super().visit_block(node)
 
     def visit_func_def(self, node: FuncDef) -> None:
         node = self.fixup(node)

test-data/unit/fine-grained.test

@@ -10155,3 +10155,53 @@ def test() -> None:
 [out]
 ==
 m.py:4: error: Argument 1 to "meth" of "C" has incompatible type "int"; expected "D"
+
+[case testNoNestedDefinitionCrash]
+import m
+[file m.py]
+from typing import Any, TYPE_CHECKING
+
+class C:
+    if TYPE_CHECKING:
+        def __init__(self, **kw: Any): ...
+
+C
+[file m.py.2]
+from typing import Any, TYPE_CHECKING
+
+class C:
+    if TYPE_CHECKING:
+        def __init__(self, **kw: Any): ...
+
+C
+# change
+[builtins fixtures/dict.pyi]
+[out]
+==
+
+[case testNoNestedDefinitionCrash2]
+import m
+[file m.py]
+from typing import Any
+
+class C:
+    try:
+        def __init__(self, **kw: Any): ...
+    except:
+        pass
+
+C
+[file m.py.2]
+from typing import Any
+
+class C:
+    try:
+        def __init__(self, **kw: Any): ...
+    except:
+        pass
+
+C
+# change
+[builtins fixtures/dict.pyi]
+[out]
+==