[NFC] Refactor MBB hotness/coldness into templated PSI functions.

In D152399, we calculate BPI->BFI in MachineFunctionSplit pass just to
use PSI->isFunctionHotInCallGraph, which is expensive. Instead, we can
implement this directly with MBFI.

Reviewer mentioned in the comment, that machine_size_opts already has
isFunctionColdInCallGraph, isFunctionHotInCallGraphNthPercentile, etc
implemented. These can be refactored and reused across MFS and machine
size opts.

This CL does this - it refactors out those internal static functions
into PSI as templated functions, so they can be accessed easily.

Differential Revision: https://reviews.llvm.org/D152758