[InstanceGraph] Expose choice of top in getAbsolutePaths, NFC.

This exposes a new getAbsolutePaths API where the user may specify an
InstanceGraphNode to use as the top of the returned paths. The
existing API that does not expose this simply calls the new API using
getTopLevelNode(), preserving the previous behavior. An upcoming
change will use the new API to get absolute paths relative to a
different top.

Author: mikeurbach

include/circt/Support/InstanceGraph.h

@@ -333,6 +333,8 @@ struct InstancePathCache {
   explicit InstancePathCache(InstanceGraph &instanceGraph)
       : instanceGraph(instanceGraph) {}
   ArrayRef<InstancePath> getAbsolutePaths(ModuleOpInterface op);
+  ArrayRef<InstancePath> getAbsolutePaths(ModuleOpInterface op,
+                                          InstanceGraphNode *top);
 
   /// Replace an InstanceOp. This is required to keep the cache updated.
   void replaceInstance(InstanceOpInterface oldOp, InstanceOpInterface newOp);

lib/Support/InstanceGraph.cpp

@@ -227,12 +227,18 @@ InstanceGraph::getInferredTopLevelNodes() {
 
 static InstancePath empty{};
 
-// NOLINTBEGIN(misc-no-recursion)
 ArrayRef<InstancePath>
 InstancePathCache::getAbsolutePaths(ModuleOpInterface op) {
+  return getAbsolutePaths(op, instanceGraph.getTopLevelNode());
+}
+
+// NOLINTBEGIN(misc-no-recursion)
+ArrayRef<InstancePath>
+InstancePathCache::getAbsolutePaths(ModuleOpInterface op,
+                                    InstanceGraphNode *top) {
   InstanceGraphNode *node = instanceGraph[op];
 
-  if (node == instanceGraph.getTopLevelNode()) {
+  if (node == top) {
     return empty;
   }