For loop assumed to execute at least once based on inferred type

**Bug Report**

Mypy seems to assume that for loop body executes at least once when inferring index variable types.

**To Reproduce**

```python
x: int | str
x = "abc"
for x in list[int]():
    pass
reveal_type(x) # int

```

**Expected Behavior**

Revealed type is `int | str`.

**Actual Behavior**

Revealed type is `int`.

**Your Environment**

Latest master.