Fix Iftype Union Codegen

Fix code generation failure for iftype with union return type

Previously, using iftype in a function that returns a union type caused an internal compiler error during code generation:

actor Main
  new create(env: Env) =>
    test[U8]()

  fun test[T: U8](): (U8 | None) =>
    iftype T <: U8 then
      0
    end

Error:
main.pony:5:3: internal failure: code generation failed for method Main_ref_test_U8_val_o

This has been fixed. Functions using iftype with union return types now compile and run correctly.