Assert in qv4jit.cpp with disabled qml-debug feature

After commit 2f784a544ec4dccaa70ee1bcec71c8e6c2bd5d99 Nodir Temirkhodjaev reports in https://codereview.qt-project.org/#/c/220419/ that we run into a failing assertion when qml-debug as feature is disabled:

ASSERT failure in Q_UNREACHABLE(): "Q_UNREACHABLE was reached", file D:\opt\qt5\qtdeclarative\src\qml\jit\qv4jit.cpp, line 116