Add a more thorough target slot analysis

[j2kun]

When running this in debug mode

bazel run --noallow_analysis_cache_discard //tools:heir-opt -- \
    --secretize=entry-function=box_blur --wrap-generic --canonicalize --cse \
    --full-loop-unroll --canonicalize --cse \
    --insert-rotate \
    $PWD/tests/simd/box_blur_4x4.mlir --debug --debug-only=insert-rotate

The logs show:

https://gist.github.com/j2kun/00d819fc79b7e1a24e29b8354c08537f

I believe this is sufficient to thread through the pass and achieve the optimal rotations, however, it will require giving up on the pattern-based implementation because I'm not aware of a way to have tablegen patterns use the results of an analysis pass. I will inquire about it though.

[AlexanderViand-Intel]

I believe this is sufficient to thread through the pass and achieve the optimal rotations

Yes, this looks really good! Not only because the taget slots match for each of the ops in an unrolled iteration of the box_blur loop nest, but also because it doesn't seem to flag any "excessive" target slots in hamming_dist 👍

[AlexanderViand-Intel]

Less of a review comment, more a question for my own benefit/understanding of the analysis framework: what's going on here with these empty implementations?

[j2kun]

I believe these hooks are for adding special behavior at control flow entry and exit points, where by default the framework would have no propagation through them. For something like a liveness analysis

https://github.com/llvm/llvm-project/blob/2a547f0f6c6b456342b9bfad38787f54f265fc96/mlir/lib/Analysis/DataFlow/LivenessAnalysis.cpp#L104-L106

I don't specifically know which arguments of the branch ops, for example, are "not forwarded" automatically. But for our purposes for now we're assuming there is no control flow.