[SYCL] Cherry-pick "Switch to use plain array in sycl::vec in more cases" to sycl-rel-6_0_0#17697

AlexeySachkov merged 2 commits intosycl-rel-6_0_0intel/llvm:sycl-rel-6_0_0from

cherry-pick-vecintel/llvm:cherry-pick-vecCopy head branch name to clipboard