[Comb] Add canonicalization for comb.truth_table

[MrRoy09]

Adds canonicalization patterns and tests for comb.truth_table:

• Folds truth tables with constant output (all true or all false entries) into hw.constant
• Simplifies truth tables that depend on a single input to the corresponding input value
• Adds tests verifying canonicalization behavior for comb.truth_table

Closes #8925

[fzi-hielscher]

Thanks! The implementation and tests look very nice. I'm just not sure whether this really should be a canonicalization pattern. Considering the exponential growth of truth tables this could very easily have a massive effect on runtime . And it doesn't make much sense to apply this pattern repeatedly in the canonicalization convergence loop since it only depends on the truth table itself. I think we should move this to a dedicated pass instead. WDYT @uenoku?

[uenoku]

Cool, thank you for adding this! LGTM!

[uenoku]

And it doesn't make much sense to apply this pattern repeatedly in the canonicalization convergence loop since it only depends on the truth table itself.

I think that's fair concern (though the table size is already 2^n so i don't think it's going to be a critical regression). Maybe we can use the same pattern for canonicalization and dedicated pass and for canonicalization it might makes sense to restrict the lut size to small (e.g. 4 inputs). WDYT? @fzi-hielscher

Alternatively we can also disable the canonicalization for negation and simply find identity patterns
00001111, 00110011 and 01010101 which may be suitable for even folder.

[fzi-hielscher]

Yeah, not touching truth tables beyond a certain limit sounds sensible to me. That being said, I don't have an idea what sizes we are dealing with in practice and what a "reasonable" limit would be.

[teqdruid]

We've been talking about introducing a '-O1' pass for the core dialect as we're definitely abusing canonicalizers for some of them. If there's any debate, this would be a good candidate for having in that pass.