[clang] [AMDGPU][SPIR-V] Fix treating SPIR-V input as the wrong LLVM-IR (PR #202986)

[Dmitry Sidorov via cfe-commits]

https://github.com/MrSidims approved this pull request.

LGTM

Confirm, that rocm-examples are once again buildable and passing using SPIR-V backend. Thanks!

https://github.com/llvm/llvm-project/pull/202986