Forbid InterConditional in PartialStruct fields (#44438)

Alternative to #44437.

base/compiler/abstractinterpretation.jl

@@ -2068,14 +2068,18 @@ function widenreturn(@nospecialize(rt), @nospecialize(bestguess), nslots::Int, s
     # and is valid and good inter-procedurally
     isa(rt, Conditional) && return InterConditional(slot_id(rt.var), rt.vtype, rt.elsetype)
     isa(rt, InterConditional) && return rt
+    return widenreturn_noconditional(rt)
+end
+
+function widenreturn_noconditional(@nospecialize(rt))
     isa(rt, Const) && return rt
     isa(rt, Type) && return rt
     if isa(rt, PartialStruct)
         fields = copy(rt.fields)
         local anyrefine = false
         for i in 1:length(fields)
             a = fields[i]
-            a = isvarargtype(a) ? a : widenreturn(a, bestguess, nslots, slottypes, changes)
+            a = isvarargtype(a) ? a : widenreturn_noconditional(widenconditional(a))
             if !anyrefine
                 # TODO: consider adding && const_prop_profitable(a) here?
                 anyrefine = has_const_info(a) ||
@@ -2091,6 +2095,7 @@ function widenreturn(@nospecialize(rt), @nospecialize(bestguess), nslots::Int, s
     return widenconst(rt)
 end
 
+
 function handle_control_backedge!(frame::InferenceState, from::Int, to::Int)
     if from > to
         if is_effect_overrided(frame, :terminates_globally)