[LLHD] Simplify single-block llhd.process terminated by llhd.halt

[mvpant]

This simplifies an llhd.process that has only a single basic block with an llhd.halt terminator.

1. Lifts constant-like yield operands of llhd.halt operation and replaces uses of llhd.process results with them directly.
2. Deduplicates the yield operands of llhd.halt.

[mvpant]

Regarding the initial process that just ends in an llhd.halt:

    %2:2 = llhd.process -> i1, i1 {
      llhd.halt %false, %reset : i1, i1
    }
    llhd.drv %reg_0, %2#0 after %0 if %2#1 : !hw.inout<i1>

This should actually be pretty straightforward to handle. I think we're just missing a folder or canonicalizer that replaces llhd.process results with the corresponding llhd.halt constant operand, if there's only a single llhd.halt in the last block.

It's important to only do this for constant operands, since the process runs only once at very beginning. This means that any non-constant value would basically be "snapshot" at that specific point in time and produced as a result from the process. This result won't change throughout the simulation, even if the original non-constant value does. Doing this only for constants works around this problem.

This is a follow up to the discussion

[fabianschuiki]

Very cool! Thanks a lot for adding this 👍 🙌

[fabianschuiki]

Want me to merge this, @mvpant?