tfunc: Constant fold isdefined for (dt::DataType).fields

In PR #53750, we stopped constant folding `isdefined` for non-`const`
fields assuming that they may revert to `undef`. Unfortunately,
this broke `fieldcount` for `Tuple`s, causing significant inference
quality degradation. Adjust `fieldcount` to avoid this.

base/reflection.jl

@@ -1003,12 +1003,15 @@ function datatype_fieldcount(t::DataType)
  return fieldcount(types)
  end
  return nothing
- elseif isabstracttype(t) || (t.name === Tuple.name && isvatuple(t))
+ elseif isabstracttype(t)
  return nothing
  end
- if isdefined(t, :types)
+ if t.name === Tuple.name
+ isvatuple(t) && return nothing
  return length(t.types)
  end
+ # Equivalent to length(t.types), but `t.types` is lazy and we do not want
+ # to be forced to compute it.
  return length(t.name.names)
 end
 

test/compiler/inference.jl

@@ -5725,3 +5725,6 @@ let interp = CachedConditionalInterp();
  result.linfo.def.name === :func_cached_conditional
  end == 1
 end
+
+# fieldcount on `Tuple` should constant fold, even though `.fields` not const
+@test fully_eliminated(Base.fieldcount, Tuple{Type{Tuple{Nothing, Int, Int}}})