[Arc] Allow top-level logic in LowerState; detect clock edges (#6142)

Make the `LowerState` pass allow operations to remain in the top-level
`arc.model` op after state lowering. This is necessary for lowering the
model op into an `eval` function in the future. Make use of this new
flexibility by inserting logic into the model that detects edges on the
clocks of the `arc.clock_tree` ops. The clock trees no longer trigger on
the clock itself, but are given an "enable" signal that indicates
whether a clock edge has been observed.

In the future, we'll want to schedule the ops in the `arc.model` and
lower it to a separate `eval` function, instead of throwing it away. In
doing so the user will no longer have to manually call clock functions,
but can call a singular `eval` function instead. A centralized function
that performs model execution will also allow us to properly simulate
clock edges occurring at the same time -- something which is impossible
today.

Together with the `arc.clock_domain` op, this `eval` function will make
the entire clock detection and grouping a performance optimization
instead of a required transformation. Theoretically, even if we did not
separate state with the same clock into dedicated clock functions, we'll
still be able to generate an `eval` function, with all logic inlined.
This will ultimately make the Arc dialect more robust and the transforms
more composable.

Author: fabianschuiki

include/circt/Dialect/Arc/ArcPasses.td

@@ -158,7 +158,7 @@ def LowerState : Pass<"arc-lower-state", "mlir::ModuleOp"> {
   let constructor = "circt::arc::createLowerStatePass()";
   let dependentDialects = [
     "arc::ArcDialect", "mlir::scf::SCFDialect", "mlir::func::FuncDialect",
-    "mlir::LLVM::LLVMDialect"
+    "mlir::LLVM::LLVMDialect", "comb::CombDialect"
   ];
 }