[NFC] Add withInexactIfNoCustomDescs helper (#7528)

There are several places in the code where we need to make a type
inexact only if custom descriptors is disabled. Add a helper to slightly
simplify those places.

Author: tlively

src/passes/GUFA.cpp

@@ -375,10 +375,8 @@ struct GUFAOptimizer
 
         auto oracleType = parent.getContents(curr).getType();
         // Exact casts are only allowed when custom descriptors is enabled.
-        if (oracleType.isExact() &&
-            !getModule()->features.hasCustomDescriptors()) {
-          oracleType = oracleType.with(Inexact);
-        }
+        oracleType =
+          oracleType.withInexactIfNoCustomDescs(getModule()->features);
         if (oracleType.isRef() && oracleType != curr->type &&
             Type::isSubType(oracleType, curr->type)) {
           replaceCurrent(Builder(*getModule()).makeRefCast(curr, oracleType));

src/passes/RemoveUnusedBrs.cpp

@@ -906,11 +906,10 @@ struct RemoveUnusedBrs : public WalkerPass<PostWalker<RemoveUnusedBrs>> {
         // further optimizations after this, and those optimizations might even
         // benefit from this improvement.
         auto glb = Type::getGreatestLowerBound(curr->castType, refType);
-        if (!getModule()->features.hasCustomDescriptors() && glb.isExact() &&
-            !curr->castType.isExact()) {
+        if (!curr->castType.isExact()) {
           // When custom descriptors is not enabled, nontrivial exact casts are
           // not allowed.
-          glb = glb.with(Inexact);
+          glb = glb.withInexactIfNoCustomDescs(getModule()->features);
         }
         if (curr->op == BrOnCastFail) {
           // BrOnCastFail sends the input type, with adjusted nullability. The

src/wasm-type.h

@@ -418,16 +418,21 @@ class Type {
   }
 
   // Return a new reference type with some part updated to the specified value.
-  Type with(HeapType heapType) {
+  Type with(HeapType heapType) const {
     return Type(heapType, getNullability(), getExactness());
   }
-  Type with(Nullability nullability) {
+  Type with(Nullability nullability) const {
     return Type(getHeapType(), nullability, getExactness());
   }
-  Type with(Exactness exactness) {
+  Type with(Exactness exactness) const {
     return Type(getHeapType(), getNullability(), exactness);
   }
 
+  // Make the type inexact if custom descriptors is not enabled.
+  Type withInexactIfNoCustomDescs(FeatureSet feats) const {
+    return !isExact() || feats.hasCustomDescriptors() ? *this : with(Inexact);
+  }
+
 private:
   template<bool (Type::*pred)() const> bool hasPredicate() {
     for (const auto& type : *this) {