[NFC] Remove StringLowering::replaceNulls (#7607)

Now that `string` is a subtype of `extern`, the null type for strings
and externrefs is the same, so we no longer need to fix up nulls in
StringLowering.

Unblocks #7601.

Author: tlively

src/passes/StringLowering.cpp

@@ -227,9 +227,6 @@ struct StringLowering : public StringGathering {
     // Replace string.* etc. operations with imported ones.
     replaceInstructions(module);
 
-    // Replace ref.null types as needed.
-    replaceNulls(module);
-
     // ReFinalize to apply all the above changes.
     ReFinalize().run(getPassRunner(), module);
 
@@ -495,61 +492,6 @@ struct StringLowering : public StringGathering {
     replacer.run(getPassRunner(), module);
     replacer.walkModuleCode(module);
   }
-
-  // A ref.null of none needs to be noext if it is going to a location of type
-  // stringref.
-  void replaceNulls(Module* module) {
-    // Use SubtypingDiscoverer to find when a ref.null of none flows into a
-    // place that has been changed from stringref to externref.
-    struct NullFixer
-      : public WalkerPass<
-          ControlFlowWalker<NullFixer, SubtypingDiscoverer<NullFixer>>> {
-      // Hooks for SubtypingDiscoverer.
-      void noteSubtype(Type, Type) {
-        // Nothing to do for pure types.
-      }
-      void noteSubtype(HeapType, HeapType) {
-        // Nothing to do for pure types.
-      }
-      void noteSubtype(Type, Expression*) {
-        // Nothing to do for a subtype of an expression.
-      }
-      void noteSubtype(Expression* a, Type b) {
-        // This is the case we care about: if |a| is a null that must be a
-        // subtype of ext then we fix that up.
-        if (!b.isRef()) {
-          return;
-        }
-        HeapType top = b.getHeapType().getTop();
-        if (top.isMaybeShared(HeapType::ext)) {
-          if (auto* null = a->dynCast<RefNull>()) {
-            null->finalize(HeapTypes::noext.getBasic(top.getShared()));
-          }
-        }
-      }
-      void noteSubtype(Expression* a, Expression* b) {
-        // Only the type matters of the place we assign to.
-        noteSubtype(a, b->type);
-      }
-      void noteNonFlowSubtype(Expression* a, Type b) {
-        // Flow or non-flow is the same for us.
-        noteSubtype(a, b);
-      }
-      void noteCast(HeapType, HeapType) {
-        // Casts do not concern us.
-      }
-      void noteCast(Expression*, Type) {
-        // Casts do not concern us.
-      }
-      void noteCast(Expression*, Expression*) {
-        // Casts do not concern us.
-      }
-    };
-
-    NullFixer fixer;
-    fixer.run(getPassRunner(), module);
-    fixer.walkModuleCode(module);
-  }
 };
 
 Pass* createStringGatheringPass() { return new StringGathering(); }