[FIRRTL] Fix InstanceOp::getPortDomain

Fix a mistake in the `InstanceOp::getPortDomain` member function where
this would always return a casted `ArrayAttr` for a port's domain
information.  This is not correct as the domain information can be
_either_ an `ArrayAttr` (if a non-domain port) or a
`FlatSymbolRefAttr` (if a domain port).

Signed-off-by: Schuyler Eldridge <[email protected]>

Author: seldridge

include/circt/Dialect/FIRRTL/FIRRTLDeclarations.td

@@ -148,7 +148,7 @@ def InstanceOp : HardwareDeclOp<"instance", [
     void setAllPortAnnotations(ArrayRef<Attribute> annotations);
 
     /// Hooks for domain information.
-    ArrayAttr getPortDomain(unsigned portIdx);
+    Attribute getPortDomain(unsigned portIdx);
 
     /// Clone this instance with inserted ports.
     InstanceOp cloneWithInsertedPorts(

lib/Dialect/FIRRTL/FIRRTLOps.cpp

@@ -2658,10 +2658,10 @@ void InstanceOp::setAllPortAnnotations(ArrayRef<Attribute> annotations) {
                    ArrayAttr::get(getContext(), annotations));
 }
 
-ArrayAttr InstanceOp::getPortDomain(unsigned portIdx) {
+Attribute InstanceOp::getPortDomain(unsigned portIdx) {
   assert(portIdx < getNumResults() &&
          "index should be smaller than result number");
-  return cast<ArrayAttr>(getDomainInfo()[portIdx]);
+  return getDomainInfo()[portIdx];
 }
 
 InstanceOp InstanceOp::cloneWithInsertedPorts(