Assignment Propagation

[Nikil-Shyamsunder]

This PR changes the graph IR assignment representation away from nested ITEs and adds a reaching-definitions pass that gives us info so we can propagates assignment forward so every reachable node has a total assignment for every DUT input port.

The main interesting things:

Assignment Representation: (see #268 for details)

• Op::Assign now stores an Assignment summary: Assignment { dont_care: ExprRef, concretes: Vec<(ExprRef, ExprRef)> }

• edge_contract.rs implements ordered and unordered assignment merging over this representation.

  • Ordered merge is used for sequential/intra-transaction merging, where late assignments win.
  • Unordered merge is used for parallel/graft/determinize-style merging, where concrete assignments from one side are preferred over DontCare from the
    other side.

• The graph interpreter and graphviz output were updated to understand the new assignment summaries.

  • Codegen/evaluation checks dont_care first, then concrete branches.
  • Graphviz prints assignments like X if g; value if h.
  • There is an internal assertion if an assignment reaches no DontCare or concrete branch.

Reaching Definitions analysis:

• reaching_defs.rs adds reaching-definitions analysis over the graph.
  • It computes out_defs per node
  • The analysis restricts incoming assignments by transition guards before merging.
  • Incoming definitions are merged with unordered semantics.
  • If a join produces multiple possible incoming outcomes for the same port, that port is marked as an incoming conflict.
  • A node-local assignment kills the incoming conflict for that port.
  • If any incoming conflicts remain after the transfer function for a node, we fail fast.

Assignment propagation:

• propagate_assigns.rs uses the reaching definitions to rewrite reachable nodes so that every DUT input port has a total assignment at every node.
  • It removes old input assignments from each reachable node.
  • It inserts the reaching out_def assignment for every input port.
  • This pass assumes the reaching-def analysis has already rejected unresolved
    incoming conflicts.

Default DontCare on Entry, Assign DontCare on exit:

• lowering.rs now emits explicit default DontCare assignments on the entry node.
  • The fragment exit also assigns every DUT input to DontCare, i.e. transactions relinquish their ports on exit.

Tests

This PR updates the unit tests around assignment merging and lowering:

• edge-contract tests now construct guarded assignments by putting guards inside Assignment, not on the Action.
• unordered merge tests now expect effective concrete guards.
• lowering tests now account for the explicit fragment-entry DontCare node and exit relinquish assignments.
• TODO: there are some Xfail tests due to waveform diffing expressivity.

[ekiwi]

@Nikil-Shyamsunder : I just rebased this on main. I will merge as soon as CI passes.