[ImportVerilog] Capture analysis should skip reprocessing identical module instances (#10338)

Currently capture analysis unconditionally descends into all module instance
bodies. This forces slang to instantiate complete ASTs for all module
instances, transitively. For large designs this does not scale. For example
with one 7M LOC Verilog example, ImportVerilog consumes more than 200GB of
memory before crashing with OOM.

Slang lets clients avoid this problem via "canonical instance bodies"
that express deduplication. Setting `VisitCanonical` to true in the
`ASTVisitor` template causes the `InstanceBodySymbol` visit to use
the deduplicated canonical instance body (when available). For this
analysis we don't need to re-analyze duplicate instance so we can
just keep track of which bodies we've visited and avoid descending
into already-visited bodies.

Author: rocallahan

lib/Conversion/ImportVerilog/CaptureAnalysis.cpp

@@ -80,7 +80,9 @@ namespace {
 /// visiting enabled so that we recurse into all function bodies.
 struct CaptureWalker
     : public ASTVisitor<CaptureWalker, /*VisitStatements=*/true,
-                        /*VisitExpressions=*/true> {
+                        /*VisitExpressions=*/true,
+                        /*VisitBad=*/false,
+                        /*VisitCanonical=*/true> {
 
   /// The function whose body we are currently inside, or nullptr if we are at
   /// a scope outside any function.
@@ -89,6 +91,8 @@ struct CaptureWalker
   /// Captured variables per function.
   CaptureMap capturedVars;
 
+  mlir::DenseSet<const InstanceBodySymbol *> visitedInstanceBodies;
+
   /// Inverse call graph: maps each callee to the set of callers that call it.
   /// Used to propagate captures from callees to their callers. Uses MapVector
   /// for deterministic iteration order during propagation.
@@ -150,6 +154,12 @@ struct CaptureWalker
     visitDefault(expr);
   }
 
+  void handle(const InstanceBodySymbol &instance) {
+    if (visitedInstanceBodies.insert(&instance).second) {
+      visitDefault(instance);
+    }
+  }
+
   /// Propagate captures transitively through the call graph. For each callee
   /// that has captures, push each captured variable upward through all
   /// transitive callers using a worklist. A captured variable is only