[NFC] Add more const annotations + a trivial ==

src/ir/possible-constant.h

@@ -48,6 +48,10 @@ struct PossibleConstantValues {
 public:
   PossibleConstantValues() : value(None()) {}
 
+  bool operator==(const PossibleConstantValues& other) const {
+    return value == other.value;
+  }
+
   // Notes the contents of an expression and update our internal knowledge based
   // on it and all previous values noted.
   void note(Expression* expr, Module& wasm) {
@@ -155,7 +159,7 @@ struct PossibleConstantValues {
   }
 
   // Assuming we have a single value, make an expression containing that value.
-  Expression* makeExpression(Module& wasm) {
+  Expression* makeExpression(Module& wasm) const {
     Builder builder(wasm);
     if (isConstantLiteral()) {
       return builder.makeConstantExpression(getConstantLiteral());

src/ir/subtypes.h

@@ -145,7 +145,8 @@ struct SubTypes {
   // Efficiently iterate on subtypes of a type, up to a particular depth (depth
   // 0 means not to traverse subtypes, etc.). The callback function receives
   // (type, depth).
-  template<typename F> void iterSubTypes(HeapType type, Index depth, F func) {
+  template<typename F>
+  void iterSubTypes(HeapType type, Index depth, F func) const {
     // Start by traversing the type itself.
     func(type, 0);
 
@@ -186,7 +187,7 @@ struct SubTypes {
   }
 
   // As above, but iterate to the maximum depth.
-  template<typename F> void iterSubTypes(HeapType type, F func) {
+  template<typename F> void iterSubTypes(HeapType type, F func) const {
     return iterSubTypes(type, std::numeric_limits<Index>::max(), func);
   }