[LLHD] Mem2Reg creates drives to read-only signals

[fabianschuiki]

The following only reads signal %a_0, but LLHD's mem2reg pass still inserts a corresponding drive. Reproduce with circt-opt --llhd-mem2reg on the following:

module {
  hw.module @Foo(in %a : i32) {
    %0 = llhd.constant_time <0ns, 0d, 1e>
    %c0_i32 = hw.constant 0 : i32
    %a_0 = llhd.sig name "a" %c0_i32 : i32
    %1 = llhd.prb %a_0 : !hw.inout<i32>
    llhd.process {
      cf.br ^bb1
    ^bb1:  // 2 preds: ^bb0, ^bb1
      %2 = llhd.prb %a_0 : !hw.inout<i32>
      func.call @dummy(%2) : (i32) -> ()
      llhd.wait (%1 : i32), ^bb1
    }
    llhd.drv %a_0, %a after %0 : !hw.inout<i32>
    hw.output
  }
  func.func private @dummy(%arg0: i32) {
    return
  }
}

The pass should probably track if a definition comes from a probe inserted by the pass, or an actual drive to the signal.