This patch extends the PGO infrastructure with an option to prefer the instrumentation of loop entry blocks. This option is a generalization of https://github.com/llvm/llvm-project/commit/19fb5b467bb97f95eace1f3637d2d1041cebd3ce, and helps to cover cases where the loop exit is never executed. An example where this can occur are event handling loops. Note that change does NOT change the default behavior.