Enable some of the existing size optimizations for cold code under PGO.
A ~5% code size saving in big internal app under PGO.
The way it gets BFI/PSI is discussed in the RFC thread
http://lists.llvm.org/pipermail/llvm-dev/2019-March/130894.html
Note it doesn't currently touch loop passes.
Perhaps better to default to false, do robust performance testing and flip the option. You've mentioned code size reduction in one binary in the description, but good to validate there are no performance regressions in spec.