[CSSPGO] Turn on priority inlining for probe-only profile

We have seen that the prioirty inliner delivered on-par performance with the old inliner for probe-only CSSPGO profile, as long as without a size budget. I'm turning on the priority inliner for probe-only profile by default.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D124632