8388706: PolynomialP256Bench::benchAssign regression after JDK-8355216

[smemery]

This fix allows for more accurate comparisons between intrinsics and Java performance for P256 conditional set operations.
Before this fix, benchmarks showed a 53% regression for intrinsics vs. Java due to constant propagation and dead-code removal by C2. After this fix, only a 8% drop in performance is observed for intrinsics vs. Java.

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Issue

• JDK-8388706: PolynomialP256Bench::benchAssign regression after JDK-8355216 (Bug - P4)

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• 02: Full - Incremental (5e7a26d2)
• 01: Full - Incremental (e9ab6be9)
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