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, 11 insertions, 2 deletions

diff --git a/test/passes/optimize-instructions_all-features.wast b/test/passes/optimize-instructions_all-features.wast
index 6e106ad1b..c2fa16e35 100644
--- a/test/passes/optimize-instructions_all-features.wast
+++ b/test/passes/optimize-instructions_all-features.wast
@@ -4207,8 +4207,17 @@
       (unreachable)
     )
   )
-  ;; Tests when if arms are subtype of if's type
-  (func $if-arms-subtype (result anyref)
+  ;; These functions test if an `if` with subtyped arms is correctly folded
+  ;; 1. if its `ifTrue` and `ifFalse` arms are identical (can fold)
+  (func $if-arms-subtype-fold (result anyref)
+    (if (result anyref)
+      (i32.const 0)
+      (ref.null extern)
+      (ref.null extern)
+    )
+  )
+  ;; 2. if its `ifTrue` and `ifFalse` arms are not identical (cannot fold)
+  (func $if-arms-subtype-nofold (result anyref)
     (if (result anyref)
       (i32.const 0)
       (ref.null extern)