Widen unions less agressively during inference

Use `typejoin` for unions growing too large instead of just returning
`Any`.

Author: martinholters

base/inference.jl

@@ -2619,9 +2619,11 @@ function tmerge(@nospecialize(typea), @nospecialize(typeb))
         end
     end
     u = Union{typea, typeb}
-    if unionlen(u) > MAX_TYPEUNION_LEN || type_too_complex(u, MAX_TYPE_DEPTH)
-        # don't let type unions get too big
-        # TODO: something smarter, like a common supertype
+    if unionlen(u) > MAX_TYPEUNION_LEN
+        u = typejoin(typea, typeb)
+    end
+    if type_too_complex(u, MAX_TYPE_DEPTH)
+        # TODO: is this needed?
         return Any
     end
     return u

test/inference.jl

@@ -1102,3 +1102,6 @@ let isa_tfunc = Core.Inference.t_ffunc_val[
         @test isa_tfunc(c, Type{Complex{T}} where T) === Const(false)
     end
 end
+
+f_ADD_PR_NUMBER(x) = (Int8(0), Int16(0), Int32(0), Int64(0))[x]
+@test Base.return_types(f_ADD_PR_NUMBER, Tuple{Int})[1] <: Signed