Make sure fieldcount constant-folds for Tuple{...} (#54239)

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

(cherry picked from commit 3d6da2c)

base/reflection.jl

@@ -1000,12 +1000,15 @@ 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

@@ -5651,3 +5651,6 @@ let t = ntuple(i -> i % 8 == 1 ? Int64 : Float64, 4000)
     @test only(Base.return_types(Base.promote_typeof, t)) == Type{Float64}
     @test only(Base.return_types(vcat, t)) == Vector{Float64}
 end
+
+# fieldcount on `Tuple` should constant fold, even though `.fields` not const
+@test fully_eliminated(Base.fieldcount, Tuple{Type{Tuple{Nothing, Int, Int}}})