inference: add missing TypeVar handling for instanceof_tfunc (#55884)

I thought these sort of problems had been addressed by d60f92c, but it
seems some were missed. Specifically, `t.a` and `t.b` from `t::Union`
could be `TypeVar`, and if they are passed to a subroutine or recursed
without being unwrapped or rewrapped, errors like #55882
could occur.

This commit resolves the issue by calling `unwraptv` in the `Union`
handling within `instanceof_tfunc`. I also found a similar issue inside
`nfields_tfunc`, so that has also been fixed, and test cases have been
added. While I haven't been able to make up a test case specifically for
the fix in `instanceof_tfunc`, I have confirmed that this commit
certainly fixes the issue reported in #55882.

- fixes #55882

Author: aviatesk

base/compiler/tfuncs.jl

@@ -134,8 +134,8 @@ function instanceof_tfunc(@nospecialize(t), @nospecialize(troot) = t)
         end
         return tr, isexact, isconcrete, istype
     elseif isa(t, Union)
-        ta, isexact_a, isconcrete_a, istype_a = instanceof_tfunc(t.a, troot)
-        tb, isexact_b, isconcrete_b, istype_b = instanceof_tfunc(t.b, troot)
+        ta, isexact_a, isconcrete_a, istype_a = instanceof_tfunc(unwraptv(t.a), troot)
+        tb, isexact_b, isconcrete_b, istype_b = instanceof_tfunc(unwraptv(t.b), troot)
         isconcrete = isconcrete_a && isconcrete_b
         istype = istype_a && istype_b
         # most users already handle the Union case, so here we assume that
@@ -536,9 +536,9 @@ add_tfunc(Core.sizeof, 1, 1, sizeof_tfunc, 1)
         end
     end
     if isa(x, Union)
-        na = nfields_tfunc(𝕃, x.a)
+        na = nfields_tfunc(𝕃, unwraptv(x.a))
         na === Int && return Int
-        return tmerge(na, nfields_tfunc(𝕃, x.b))
+        return tmerge(𝕃, na, nfields_tfunc(𝕃, unwraptv(x.b)))
     end
     return Int
 end

test/compiler/inference.jl

@@ -5161,3 +5161,6 @@ let x = 1, _Any = Any
     foo27031() = bar27031((x, 1.0), Val{_Any})
     @test foo27031() == "OK"
 end
+
+issue55882_nfields(x::Union{T,Nothing}) where T<:Number = nfields(x)
+@test only(Base.return_types(issue55882_nfields)) <: Int