[FuncSpec] Use std::pow instead of operator^

The original implementation calculating UserBonus uses operator ^, which means XOR in C++
language.
At the first glance of reviewing, I thought it should be power, my bad.
It doesn't make sense to use XOR here. So I believe it should be a
carelessness as I made.

Test Plan: check-all

Reviewed By: SjoerdMeijer

Differential Revision: https://reviews.llvm.org/D104282