Preprations for controlflow fusion

[ShangkunLi]

In this pr:

• Prepare different control flow kernels for the control flow fusion test in test/controflow-fuse/ (e.g., (non)-perfect nested loops, complex nested loop, nested-reduction loop)
• Fix some bugs when trying to lower a non-perfect nested loop to dataflow
• Provide both source .cpp and .mlirfiles of the tests

[tancheng]

Thanks @ShangkunLi, it looks like we need to have our interpreter be done ASAP to verify the upcoming transformations.

[ShangkunLi]

Thanks @ShangkunLi, it looks like we need to have our interpreter be done ASAP to verify the upcoming transformations.

Agree. For now, I just manually go through each line of the generated dataflow ir to ensure the correctness.