Allow external lattice elements to properly union split

Currently `MustAlias` is the only lattice element that is allowed
to widen to union types. However, there are others in external
packages. Expand the support we have for this in order to allow
union splitting of lattice elements.

base/compiler/typelattice.jl

@@ -120,6 +120,8 @@ end
 MustAlias(var::SlotNumber, @nospecialize(vartyp), fldidx::Int, @nospecialize(fldtyp)) =
     MustAlias(slot_id(var), vartyp, fldidx, fldtyp)
 
+_uniontypes(x::MustAlias, ts) = _uniontypes(widenconst(x), ts)
+
 """
     alias::InterMustAlias
 

base/compiler/typeutils.jl

@@ -231,8 +231,7 @@ function unionsplitcost(argtypes::Union{SimpleVector,Vector{Any}})
     nu = 1
     max = 2
     for ti in argtypes
-        # TODO remove this to implement callsite refinement of MustAlias
-        if isa(ti, MustAlias) && isa(widenconst(ti), Union)
+        if !isvarargtype(ti)
             ti = widenconst(ti)
         end
         if isa(ti, Union)
@@ -268,15 +267,18 @@ function _switchtupleunion(t::Vector{Any}, i::Int, tunion::Vector{Any}, @nospeci
     else
         origti = ti = t[i]
         # TODO remove this to implement callsite refinement of MustAlias
-        if isa(ti, MustAlias) && isa(widenconst(ti), Union)
-            ti = widenconst(ti)
-        end
         if isa(ti, Union)
             for ty in uniontypes(ti::Union)
                 t[i] = ty
                 _switchtupleunion(t, i - 1, tunion, origt)
             end
             t[i] = origti
+        elseif !isa(ti, Const) && !isvarargtype(ti) && isa(widenconst(ti), Union)
+            for ty in uniontypes(ti)
+                t[i] = ty
+                _switchtupleunion(t, i - 1, tunion, origt)
+            end
+            t[i] = origti
         else
             _switchtupleunion(t, i - 1, tunion, origt)
         end