[SYCL][AMDGPU] Set amdgpu-flat-work-group-size for SYCL reqd_work_group_size#22452

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wenju-he:fix-CodeGenSYCL-reqd-work-group-size.cppwenju-he/llvm:fix-CodeGenSYCL-reqd-work-group-size.cppCopy head branch name to clipboard