Fix RefNull issues

[dcodeIO]

• 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.

Fixes #3119

[dcodeIO]

Invocations so far:
   FuzzExec: 44857
   CompareVMs: 12290
   CheckDeterminism: 3898
   Wasm2JS: 10750
   Asyncify: 11362

ITERATION: 52628

[tlively]

LGTM with @aheejin's comment about the test addressed.