[CSSPGO] Tweak ICP threshold in top-down inliner

This change slightly relaxed the current ICP threshold in top-down inliner, specifically always allow one ICP for it. It shows some perf improvements on SPEC and our internal benchmarks. Also renamed the previous flag. We can also try to turn off PGO ICP in the future.

Reviewed By: wenlei, hoy, wmi

Differential Revision: https://reviews.llvm.org/D106588