Fix backend checks in spirv_param_decorations_quals.ll (#3988)
llvm/llvm-project#215790 removed the backend's Volatile parameter
decoration
derived from `!kernel_arg_type_qual`, so the `llc` legs can no longer
share
`CHECK-SPV-IR` with the translator legs, which still expect it.
Give them a `CHECK-BACKEND` prefix matching current output, keeping the
cross-compilation coverage from #3564. Fixes the nightly `main` failure.
Original commit:
https://github.com/KhronosGroup/SPIRV-LLVM-Translator/commit/f30f8157d87aced
