This PR fixes https://github.com/llvm/llvm-project/issues/122075. We
prefer SPV_INTEL_optnone over SPV_EXT_optnone when both extensions are
available, otherwise, when a target specifies a required extension
explicitly rather than allowing any of those (e.g., by providing
--spirv-ext=all command line argument), the Backend's behavior remains
unchanged. An existing test case is updated to check the case of 2
alternative extensions available at the same time.
86440cbc74