Prevent to type ivar having initializer as nilable on assignment (#5120)

Fixed #5112

spec/compiler/semantic/initialize_spec.cr

@@ -750,4 +750,22 @@ describe "Semantic: initialize" do
       Bar.new
     )) { types["Bar"] }
   end
+
+  it "doesn't type ivar having initializer as nilable even if it is used before assigned inside initialize (#5112)" do
+    assert_type(%(
+      class Foo
+        @x = 42
+
+        def initialize
+          @x = x
+        end
+
+        def x
+          @x
+        end
+      end
+
+      Foo.new.x
+      )) { int32 }
+  end
 end

src/compiler/crystal/semantic/main_visitor.cr

@@ -839,7 +839,7 @@ module Crystal
 
         # Check if an instance variable is being assigned (for the first time)
         # and self, or that same instance variable, was used (read) before that.
-        unless @vars.has_key?(var_name)
+        unless @vars.has_key?(var_name) || scope.has_instance_var_initializer?(var_name)
           if (found_self = @found_self_in_initialize_call) ||
              (used_ivars_node = @used_ivars_in_calls_in_initialize.try(&.[var_name]?)) ||
              (@block_nest > 0)