[FIRRTL] Add GroupSink pass

[seldridge]

Add a new pass, GroupSink/firrtl-group-sink, which will sink operations that are only used by optional groups into optional groups.

[seldridge]

This is currently a very basic implementation that relies on MLIR's controlFlowSink to sink operations into specified regions (the groups!). This works for nodes and primitive operations. It expectedly misses all the fun stuff that SVExtractTestCode does related to wires/registers and instances. This needs to be beefed up with support for backedge analysis to figure out how to move any operation which isn't used outside a root group into a group.

[seldridge]

I was originally going to keep this in draft until this had a more advanced sinking approach (SVExtractTestCode's backedge tracking). However, it's useful to get something in on top of which I or others can jump in and iterate.

Here is a simple end-to-end example showing what this can do:

Input:

FIRRTL version 3.1.0
circuit Foo: %[[
  {"class":"firrtl.transforms.DontTouchAnnotation", "target":"~Foo|Foo>b"},
  {"class":"firrtl.transforms.DontTouchAnnotation", "target":"~Foo|Foo>c"}
]]
  declgroup A, bind:

  module Foo:
    input a: UInt<1>

    node b = a

    group A:
      node c = b

Output is below. Notice how wire b is moved inside Foo_A:

// Generated by CIRCT firtool-1.53.0-205-g070970d77-dirty
module Foo_A(
  input _a
);

  wire b = _a;
  wire c = b;
endmodule

module Foo(
  input a
);

endmodule


// ----- 8< ----- FILE "groups_Foo_A.sv" ----- 8< -----

// Generated by CIRCT firtool-1.53.0-205-g070970d77-dirty
`ifndef groups_Foo_A
`define groups_Foo_A
bind Foo Foo_A foo_A (
  ._a (a)
);
`endif // groups_Foo_A

[seldridge]

This will block sinking of any operation which has an inner symbol or DontTouchAnnotation. This behavior is conservative and likely can be relaxed.