[flang] Correct folding of SAME_TYPE_AS()

[klausler]

Summary

The result can't be known to be true at compilation time when
either operand is polymorphic.

[clementval]

LGTM

flang/lib/Evaluate/type.cpp

 bool DynamicType::IsTkLenCompatibleWith(const DynamicType &that) const {
   return AreCompatibleTypes(*this, that, false, false);
 }
 
 // 16.9.165
 std::optional<bool> DynamicType::SameTypeAs(const DynamicType &that) const {
   bool x{AreCompatibleTypes(*this, that, true, true)};
   bool y{AreCompatibleTypes(that, *this, true, true)};
-  if (x == y) {
-    return x;
+  if (!x && !y) {
+    return false;
+  } else if (x && y && !IsPolymorphic() && !that.IsPolymorphic()) {
+    return true;
   } else {
-    // If either is unlimited polymorphic, the result is unknown.
     return std::nullopt;
   }
 }
 
 // 16.9.76
 std::optional<bool> DynamicType::ExtendsTypeOf(const DynamicType &that) const {
   if (IsUnlimitedPolymorphic() || that.IsUnlimitedPolymorphic()) {
     return std::nullopt; // unknown