[SPIRV] Do not emit @llvm.used (#179498)
Extending the work from https://github.com/llvm/llvm-project/pull/162678
which skips `llvm.compiler.used`, we also need to skip processing of
`llvm.used`.
The OpenMP frontend puts an `addrspace(2)` variable in `llvm_used`, but
the `llvm.used` array type uses the generic AS `addrspace(4)` so there's
a `addrspacecast` from `2` to `4` inside the initalizer for `llvm.used`
which is illegal and errors in the backend.
There is
[discussion](https://github.com/llvm/llvm-project/pull/162678#issuecomment-3396735124)
in the above linked PR that the handing should be the same for
`llvm.used` and `llvm.compiler.used`, but `llvm.used` was not added to
minimize the scope of the PR.
This is required to use the OpenMP Device RTL with SPIR-V.
Signed-off-by: Nick Sarnie <nick.sarnie@intel.com>
