[llvm] [Instrumentation] Fix EdgeCounts vector size in SetBranchWeights (PR #99064)

[Mircea Trofin via llvm-commits]

mtrofin wrote:

> Thanks a lot @ellishg and @mtrofin for your help and guidance.
> 
> Can this please be backported to release/19.x?

@tstellar for the release bit. @tstellar happy to help, is this more than cherrypicking to a particular branch?

https://github.com/llvm/llvm-project/pull/99064