inference: stronger state update from branching

This change addresses
<#40832 (review)>

test/compiler/inference.jl

@@ -2207,22 +2207,34 @@ end |> only === String
     c === 42 ? :a : "b"
 end |> only === String
 
-@testset "conditional constraint propagation from non-`Conditional` object" begin
-    @test Base.return_types((Bool,)) do b
+@testset "state update on branching" begin
+    # refine condition type into constant boolean value on branching
+    @test Base.infer_return_type((Bool,)) do b
         if b
             return !b ? nothing : 1 # ::Int
         else
             return 0
         end
-    end == Any[Int]
+    end == Int
 
-    @test Base.return_types((Any,)) do b
+    # even when the original type isn't boolean type
+    @test Base.infer_return_type((Any,)) do b
         if b
             return b # ::Bool
         else
             return nothing
         end
-    end == Any[Union{Bool,Nothing}]
+    end == Union{Bool,Nothing}
+
+    # even when the original type is `Conditional`
+    @test Base.infer_return_type((Any,)) do a
+        b = isa(a, Int)
+        if b
+            return !b ? nothing : a # ::Int
+        else
+            return 0
+        end
+    end == Int
 end
 
 @testset "`from_interprocedural!`: translate inter-procedural information" begin