[mypyc] Fixing index variable in for-loop with builtins.enumerate.

[advait-dixit]

Fixes mypyc/mypyc#1046

This change fixes two problems:

1. The index variable was getting instantiated even while enumerating an empty iterable.
2. After exiting the for-loop, the value of the index variable is off by 1 (see issue linked above).

This change fixes both problems by assigning the temporary register to the index variable at the beginning of the for-loop body. Before this change, this assignment was happening before the for-loop and at the end of the for-loop body.

[hauntsaninja]

Nice, that's a bad correctness bug!