[llvm] [NFC][SPIRV] New test for untested SPIRV backend case (PR #185686)

[Dmitry Sidorov via llvm-commits]

MrSidims wrote:

@ambergorzynski  please shorten PR description, so when we merge with squash and merge we won't incidentally introduce overwhelming commit message to git log.

https://github.com/llvm/llvm-project/pull/185686