[mypyc] Fix for loop over range mutating start variable (#8672)

Don't modify the range start variable in a loop like this:

```
def f() -> None:
    n = 0
    for x in range(n, 5):
        pass
    print(n)  # Should be 0, not 5
```

Author: JukkaL

mypyc/irbuild/for_helpers.py

@@ -441,7 +441,9 @@ def init(self, start_reg: Value, end_reg: Value, step: int) -> None:
         self.end_reg = end_reg
         self.step = step
         self.end_target = builder.maybe_spill(end_reg)
-        self.index_reg = builder.maybe_spill_assignable(start_reg)
+        index_reg = builder.alloc_temp(start_reg.type)
+        builder.assign(index_reg, start_reg, -1)
+        self.index_reg = builder.maybe_spill_assignable(index_reg)
         # Initialize loop index to 0. Assert that the index target is assignable.
         self.index_target = builder.get_assignment_target(
             self.index)  # type: Union[Register, AssignmentTarget]

mypyc/test-data/run.test

@@ -51,6 +51,7 @@ def count(n: int) -> None:
 def count_between(n: int, k: int) -> None:
     for i in range(n, k):
         print(i)
+    print('n=', n)
 def count_down(n: int, k: int) -> None:
     for i in range(n, k, -1):
         print(i)
@@ -115,9 +116,11 @@ str_iter("abc")
 12
 13
 14
+n= 11
 100000000000000000000
 100000000000000000001
 100000000000000000002
+n= 100000000000000000000
 20
 19
 18