DO NOT MERGE: Remove ineffective distribute-constants optimization

[sjalander]

The distributed-constants optimization has no effect on the cycle count for the hls test suite. So, it is being removed.

[haved]

Woah, I tried replacing it with NodeReduction and got errors, I had not considered just downright removing it.

I think the reason there was a dedicated "move constants in" pass was to avoid buffers holding constant values being created, e.g. before thetas. Do we have any quick way of determining if this change has a drastic effect on the size of the HLS?

[sjalander]

I don't think we currently have any quick way of doing it. But I guess it wouldn't be difficult to count nodes and different types of nodes in the final graph and compare numbers.

[sjalander]

@haved Adding the NodeReduction before mem_sep_argument() causes an assert to be triggered similarly if it is placed before mem_queue(). I've not debugged these to find out what assumptions that break.
It is possible to add the NodeReduction after memstate_conv(), but without adding the statistics mentioned earlier, it's not that easy to say if it has a benefit.

[sjalander]

@haved
I also replaced the unused state with llvm::InvariantValueRedirection.
Can you check with your new statistics option to see if this has an impact on the number of nodes.

I was not able to eliminate the code completely, as the remove_unused_state() is also used in the mem_conv() pass, and simply replacing it there caused logical faults.

[github-actions]

The execution time of dynamatic/kernel_2mm has changed
  Golden cycle time: 24621
  Simulated cycles: 24701

[github-actions]

The execution time of dynamatic/kernel_2mm has changed
  Golden cycle time: 24621
  Simulated cycles: 24701

[haved]

@sjalander @phate
Among all 67 files in the hls test suite, the changes made in this PR only affects the node count in one of them: insertion_sort.c. The change is as follows:

Differences in file insertion_sort.txt:
Total change in node count: +3 (before 333)
HLS_BUF_P_10: +1 (before: 148)
HLS_BRANCH: +1 (before: 31)
HLS_SINK: +1 (before: 13)

[github-actions]

The execution time of base/test_return has changed
  Golden cycle time: 2
  Simulated cycles: 3

[github-actions]

The execution time of base/test_return has changed
  Golden cycle time: 2
  Simulated cycles: 3