compile_lll.py: buggy translation of repeat when #iteration <= 0

[daejunpark]

What's your issue about?

If the #iteration of repeat loop is less than or equal to 0, it replaces it by 2, which perhaps should not, while 1 would be more reasonable:
https://github.com/ethereum/viper/blob/b1fe12ddd8706ef3340a103916e3d3f2cc95ceef/viper/compile_lll.py#L96

Fortunately, for now, the repeat loops will be rejected in the preprocess phase if the #iteration is less than or equal to 0, but it would be good to fix for later:
https://github.com/ethereum/viper/blob/b1fe12ddd8706ef3340a103916e3d3f2cc95ceef/viper/parser/parser_utils.py#L113-L114

How can it be fixed?

Change the following line:
https://github.com/ethereum/viper/blob/b1fe12ddd8706ef3340a103916e3d3f2cc95ceef/viper/compile_lll.py#L96
to something like:

loops = num_to_bytearray(code.args[2].value) or [1]

or

loops = num_to_bytearray(code.args[2].value) or raise Exception(...)

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