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

(cherry picked from commit 4b27a16)

Author: aviatesk

base/compiler/tfuncs.jl

@@ -134,8 +134,8 @@ function instanceof_tfunc(@nospecialize(t), astag::Bool=false, @nospecialize(tro
         end
         return tr, isexact, isconcrete, istype
     elseif isa(t, Union)
-        ta, isexact_a, isconcrete_a, istype_a = instanceof_tfunc(t.a, astag, troot)
-        tb, isexact_b, isconcrete_b, istype_b = instanceof_tfunc(t.b, astag, troot)
+        ta, isexact_a, isconcrete_a, istype_a = instanceof_tfunc(unwraptv(t.a), astag, troot)
+        tb, isexact_b, isconcrete_b, istype_b = instanceof_tfunc(unwraptv(t.b), astag, troot)
         isconcrete = isconcrete_a && isconcrete_b
         istype = istype_a && istype_b
         # most users already handle the Union case, so here we assume that
@@ -549,9 +549,9 @@ 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

@@ -5682,3 +5682,6 @@ end
 t155751 = Union{AbstractArray{UInt8, 4}, Array{Float32, 4}, Grid55751{Float32, 3, _A} where _A}
 t255751 = Array{Float32, 3}
 @test Core.Compiler.tmerge_types_slow(t155751,t255751) == AbstractArray # shouldn't hang
+
+issue55882_nfields(x::Union{T,Nothing}) where T<:Number = nfields(x)
+@test Base.infer_return_type(issue55882_nfields) <: Int