IRStructurizer: Support native Julia `for` loop syntax

[maleadt]

Julia's native for k in start:stop syntax generates IR that the structurizer does not recognize.

Current workaround: Use explicit while-loop counting, which the structurizer upgrades to ForOp:

# Instead of:
for k in 1:stride:limit
    # ...
end

# Use:
k = 1
while k < limit
    # ...
    k += stride
end

Fixing this is hard because the Julia iterator protocol generates a lot of code that isn't easy to pattern match, especially after optimization.