Widen unions less agressively during inference

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

Author: martinholters

base/inference.jl

@@ -2810,9 +2810,12 @@ 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)
+        # helps convergence speed (large types that are changing value become very slow to
+        # work with very quickly)
         return Any
     end
     return u

test/inference.jl

@@ -1291,3 +1291,6 @@ let T1 = Array{Float64}, T2 = Array{_1,2} where _1
     rt = Base.return_types(g, (Union{Ref{Array{Float64}}, Ref{Array{Float32}}},))[1]
     @test rt >: Union{Type{Array{Float64}}, Type{Array{Float32}}}
 end
+
+f_23077(x) = (Int8(0), Int16(0), Int32(0), Int64(0))[x]
+@test Base.return_types(f_23077, Tuple{Int})[1] <: Signed