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.
ronryvchin
ff281f7d37