Fix RefNull issues (#3123)

* ExpressionAnalyzer: Fix `ref.null ht` equality check to include `ht`.
* Precompute: Fix `ref.null ht` expression reuse to also update `ht`.
* Fuzzing: Fix `ref.null func` becoming canonicalized to `ref.func $funcref`
  when evaluating execution results, by adding a check for `isNull`.
* Fuzzing: Print actual and expected execution results when aborting.
* Tests: Update `if-arms-subtype` test in `optimize-instructions` to check
  that identical `if` arms become folded while not identical arms are kept.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/tools/execution-results.h b/src/tools/execution-results.h
index c41b0c208..0881163f1 100644
--- a/src/tools/execution-results.h
+++ b/src/tools/execution-results.h
@@ -73,7 +73,7 @@ struct ExecutionResults {
           // change (after duplicate function elimination or roundtripping)
           // while the function contents are still the same
           for (Literal& val : ret) {
-            if (val.type == Type::funcref) {
+            if (val.type == Type::funcref && !val.isNull()) {
               val = Literal::makeFunc(Name("funcref"));
             }
           }
@@ -112,7 +112,8 @@ struct ExecutionResults {
       }
       std::cout << "[fuzz-exec] comparing " << name << '\n';
       if (results[name] != other.results[name]) {
-        std::cout << "not identical!\n";
+        std::cout << "not identical! " << results[name]
+                  << " != " << other.results[name] << "\n";
         return false;
       }
     }