Summary:
BottomNBits() - there is a single fast instruction for this on x86 since BMI2, but testing with godbolt indicates you need at least GCC 10 for the compiler to choose that instruction from the obvious C++ code. https://godbolt.org/z/5a7Ysd41h

BitwiseAnd() - this is a convenience function that works around the language flaw that the type of the result of x & y is the larger of the two input types, when it should be the smaller. This can save some ugly static_cast.

I expect to use both of these in coming HyperClockCache developments, and have applied them in a couple of places in existing code.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11660

Test Plan: unit tests added

Reviewed By: jowlyzhang

Differential Revision: D47935531

Pulled By: pdillinger

fbshipit-source-id: d148c43a1e51df4a1c549b93aaf2725a3f8d3bd6