[RISC-V] Skip ComputeReturnFlags in ArgIteratorTemplate::HasRetBuffArg

ComputeReturnFlags on RISC-V is complicated due to ABI rules on enregistering small structs and it can even potentially trigger an unnecessary class load. So take a shortcut when we only need to check whether a struct is returned in a buffer.

src/coreclr/vm/callingconvention.h

@@ -419,7 +419,26 @@ class ArgIteratorTemplate : public ARGITERATOR_BASE
     {
         WRAPPER_NO_CONTRACT;
         if (!(m_dwFlags & RETURN_FLAGS_COMPUTED))
+        {
+#ifdef TARGET_RISCV64
+            // On RISC-V computing all return flags is complicated due to ABI rules on enregistering small structs
+            // so just check struct size and be done with it.
+            TypeHandle valueType;
+            switch (this->GetReturnType(&valueType))
+            {
+                case ELEMENT_TYPE_TYPEDBYREF:
+                    return (sizeof(TypedByRef) > ENREGISTERED_RETURNTYPE_INTEGER_MAXSIZE);
+                case ELEMENT_TYPE_VALUETYPE:
+                    assert(!valueType.IsNull());
+                    assert(!valueType.IsTypeDesc());
+                    return (valueType.GetSize() > ENREGISTERED_RETURNTYPE_INTEGER_MAXSIZE);
+                default:
+                    return false;
+            }
+#else
             ComputeReturnFlags();
+#endif
+        }
         return (m_dwFlags & RETURN_HAS_RET_BUFFER);
     }