JIT: don't try fetching fields of ref classes as SPMI extra queries (#75500)

Enumeration of ref class fields requires walking up the type hierarchy.
Since this is done in an optional query to extend the viability of an
SPMI collection, skip for now.

Fixes #74968.

Author: AndyAyersMS

src/coreclr/jit/lclvars.cpp

@@ -3115,11 +3115,21 @@ void Compiler::makeExtraStructQueries(CORINFO_CLASS_HANDLE structHandle, int lev
         // outside of the current version bubble.
         return;
     }
-    unsigned fieldCnt = info.compCompHnd->getClassNumInstanceFields(structHandle);
+
+    unsigned const fieldCnt = info.compCompHnd->getClassNumInstanceFields(structHandle);
     impNormStructType(structHandle);
 #ifdef TARGET_ARMARCH
     GetHfaType(structHandle);
 #endif
+
+    // Bypass fetching instance fields of ref classes for now,
+    // as it requires traversing the class hierarchy.
+    //
+    if ((typeFlags & CORINFO_FLG_VALUECLASS) == 0)
+    {
+        return;
+    }
+
     for (unsigned int i = 0; i < fieldCnt; i++)
     {
         CORINFO_FIELD_HANDLE fieldHandle      = info.compCompHnd->getFieldInClass(structHandle, i);