[Benchmark X64] [danmoseley] Remove redundant Atomic wrapper and fix shared-prefix extraction in regex reduction

Arguments: benchmark Regex` twice on the same commit to check reproducibility. Also independently verified that the source generator produces **byte-identical output** for every benchmark pattern × options combination before and after the PR (by running the generator on both code paths locally and comparing character counts).

• Artifacts:
• BDN_Artifacts.zip (1 MB)
• results.md (137 KB)
• logs.txt (5 MB)