[llvm] [MathExtras] Optimize SaturatingAdd. Add specialized overload with reference for flag (PR #190479)

[Max Graey via llvm-commits]

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MaxGraey wrote:

Btw this vararg overload uses only in test cases. If we remove it, we won't need the pointer overload either.

https://github.com/llvm/llvm-project/pull/190479