[ImportVerilog] Implement procedural continuous assignment

[Arya-Golkari]

This PR supports lowering procedural continuous assignment statements (i.e. using the assign, deassign, force, and release keywords) into Moore.

I'm not very confident in my work, so I'd really appreciate help from reviewers! @Scheremo @fabianschuiki

[fabianschuiki]

Hey @Arya-Golkari, thanks for taking a stab at this! I'm wondering if we should create dedicated Moore dialect ops for these procedural assignments. They are pretty weird: if a procedure contains an assign ...; statement, that assignment behaves like a non-procedural/continuous assignment directly in the moore.module, but with an enable condition that is controlled procedurally. Maybe something like this?

module Foo(input int y);
  int x;
  initial begin
    #1;
    assign x = 42;
    #1;
    assign x = y;
    #1;
    deassign x;
    #1;
  end
endmodule

I would expect this to have x not driven in the first time step, then driven to 42 in the second time step, then continuously driven to whatever value y is in the third timestep, and then not driven at all again in the fourth timestep. We might be able to emit this by separating the procedural part that decides if and what to drive, from the actual assignment itself:

moore.module @Foo(in %y: i32) {
  %x = moore.variable : i32
  %proc_assign_enable = moore.variable 0 : i1
  %proc_assign_value = moore.variable 0 : i32
  moore.cond_assign %x, read(%proc_assign_value), read(%proc_assign_enable)
  moore.procedure initial {
    // wait
    moore.blocking_assign %proc_assign_value, 42
    moore.blocking_assign %proc_assign_enable, 1
    // wait
    // I don't remember if semantics are to assign the value y has at the time of the
    // procedural assignment, or if any changes in y while the assignment is active
    // should propagate to x. If the latter is the case, we'll need a multiplexer in the
    // module which we can switch over to continuously read y and feed it into the
    // cond_assign.
    moore.blocking_assign %proc_assign_value, read(%y)
    moore.blocking_assign %proc_assign_enable, 1
    // wait
    moore.blocking_assign %proc_assign_enable, 0
    // wait
  }
}

If we create dedicated procedural assign/deassign ops, we'll have to add a transformation pass that pretty much immediately removes them again and turns them roughly into the above. WDYT?

[TaoBi22]

Looks good, just a few docs comments!

[TaoBi22]

Nit: could we move this up to the first sentence to make the semantic difference a bit clearer?

[TaoBi22]

Looks like there's some missing text here?

[TaoBi22]

As above, could we clarify the semantic difference between this and deassign in this paragraph?

[TaoBi22]

Nit: we can drop the braces here