Allow repeat opcode to be set programmatically

[partylikeits1983]

Feature description

Currently the repeat opcode in miden assembly requires the developer to specify the number of loop iterations:

repeat.5
    # 5 loop iterations
end

However, it would be extremely useful to allow the number of loop iterations to be set programmatically. For example:

push.5
repeat
     # 5 loop iterations
end

Why is this feature needed?

I know that there is a while opcode where a similar functionality could be achieved, however, allowing the repeat opcode to be set programmatically would be useful.