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

[Avi Kivity via llvm-commits]

avikivity wrote:

Update:
 - added a (hopefully helpful) comment to explain why EdgeCounts is sizes differently than BBCountInfo.OutEdges()
 - added an assertion about the two sizes.

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