Fix cleanup of one-to-many assignment with untyped RHS

spec/compiler/semantic/no_return_spec.cr

@@ -327,4 +327,25 @@ describe "Semantic: NoReturn" do
       typeof(raise("").foo)
       )) { no_return.metaclass }
   end
+
+  it "types as NoReturn if followed by one-to-many assignment (#15638)" do
+    assert_type(<<-CRYSTAL) { bool }
+      def foo(x)
+        {'a', ""}
+      end
+
+      def raise(msg)
+        while true
+        end
+      end
+
+      def bar
+        x = 1
+        return true if x.is_a?(Int32)
+        a, b = foo(x)
+      end
+
+      bar
+      CRYSTAL
+  end
 end

src/compiler/crystal/semantic/cleanup_transformer.cr

@@ -379,7 +379,14 @@ module Crystal
         # `temp_assign` is this whole Assign node and its deduced type is same
         # as the original RHS's type
         temp_assign = expanded.as(Expressions).expressions.first
-        type = temp_assign.type
+        type = temp_assign.type?
+
+        # if the Assign node's RHS is untyped, this and all following
+        # assignments are unreachable
+        unless type
+          return untyped_expression node
+        end
+
         target_count = node.targets.size
         has_strict_multi_assign = @program.has_flag?("strict_multi_assign")