[SYCL] Disable dead arg elimination for free function kernels

[lbushi25]

This PR disables dead argument elimination for free function kernels. When using a free function kernel, the user sets the arguments manually using handler::set_arg but if certain arguments are optimized away in case they are not used, the implementation needs to be aware of this and remove the relevant set_arg calls. Instead, its much more feasible to just disable dead arg elimination in this case.

[AlexeySachkov]

We need a LIT test for the change, see llvm/test/Transforms/DeadArgElim/sycl-kernels.ll for an example of existing test

[aelovikov-intel]

I've removed reference to the internal bug report from the PR's description. That's not how upstream development should be done.

That said, the rest of the description was great and provides enough context for the change, so no additional actions needed.

[aelovikov-intel]

Something is wrong with codeownership. I'd expect llvm to be owned by @intel/dpcpp-tools-reviewers , not by @intel/llvm-reviewers-runtime .

[premanandrao]

Could you add a non-free-function kernel to this test and check that DAE still works?

[lbushi25]

I refactored the tests. I deleted this one and I added two test cases for free function kernels in the existing test sycl-kernels-neg.ll using LLVM IR directly.
Note that there is already another test sycl-kernels.ll in the same directory which checks that DAE still works with other kernels.