[llvm] [InstrProf] Support conditional counter updates (PR #102542)

[Alan Zhao via llvm-commits]

alanzhao1 wrote:

> > reduce the write contention by first checking whether the counter is set before overwriting it.
> 
> any result to share on runtime overhead?

On Chrome's unit tests, this change can reduce the runtime by > 4.5x: https://discourse.llvm.org/t/rfc-single-byte-counters-for-source-based-code-coverage/75685/10?u=gulfemsavrun

https://github.com/llvm/llvm-project/pull/102542