[LLHD] Deseq mistakenly marks `icmp ne %false, %false` as poison

[fabianschuiki]

Consider the following input:

hw.module @Foo(in %clk: i1, in %d: i7) {
  %0 = llhd.constant_time <0ns, 1d, 0e>
  %c0_i7 = hw.constant 0 : i7
  %true = hw.constant true
  %false = hw.constant false
  %q = llhd.sig %c0_i7 : i7
  %1:2 = llhd.process -> i7, i1 {
    cf.br ^bb1(%clk, %c0_i7, %false : i1, i7, i1)
  ^bb1(%2: i1, %3: i7, %4: i1):  // 4 preds: ^bb0, ^bb2, ^bb3, ^bb3
    llhd.wait yield (%3, %4 : i7, i1), (%clk : i1), ^bb2(%2 : i1)
  ^bb2(%5: i1):  // pred: ^bb1
    %6 = comb.xor bin %5, %true : i1
    %7 = comb.and bin %6, %clk : i1
    cf.cond_br %7, ^bb3, ^bb1(%clk, %c0_i7, %false : i1, i7, i1)
  ^bb3:  // pred: ^bb2
    %8 = comb.icmp ne %false, %false : i1
    %9 = comb.icmp ne %true, %true : i1
    %10 = comb.xor %8, %9 : i1
    cf.cond_br %10, ^bb1(%clk, %c0_i7, %true : i1, i7, i1), ^bb1(%clk, %d, %true : i1, i7, i1)
  }
  llhd.drv %q, %1#0 after %0 if %1#1 : !hw.inout<i7>
}

Running this through circt-opt --llhd-deseq --debug-only=llhd-deseq produces the following:

- Analyzing llhd.drv %q, %1#0 after %0 if %1#1 : !hw.inout<i7>
- Boolean %true: true
- Successor boolean ^bb3#succ0 -> ^bb1#arg2 = true
- Block condition ^bb2: true
- Boolean %6: !a1
- Boolean %7: !a1&a2
- Successor condition ^bb2#succ0 -> ^bb3 = !a1&a2
- Block condition ^bb3: !a1&a2
- Boolean %false: false
- Boolean %8: poison
- Boolean %9: poison
- Boolean %10: poison
- Successor condition ^bb3#succ0 -> ^bb1 = poison
- Boolean %4: poison
- Aborting loc("debug.mlir":21:3): poison condition

The values %8, %9, and %10 get mistakenly marked as poison values. Only operations on the triggers (%clk in this case) should have their value poisoned if the Deseq pass does not support mapping them to a truth table.