[SYCL][NFC] Fix check-work-group-attributes-match.cpp for AMDGPU (#22446)
b3ca5fb8a7b5 introduced amdgpu-flat-work-group-size attributes with
per-kernel values, causing each kernel to get a distinct attribute group.
Fixes CodeGenSYCL/check-work-group-attributes-match.cpp
CMPLRLLVM-76303
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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
(cherry picked from commit 7a41763b2905556b072485ec50d4f11090bd6f81)