[CodeGen] Separate MachineFunctionSplitter logic for different profile types.

In D152577 @xur has a post-submit comment regarding to an awkward usage
of MFS for Autofdo - instead of just using -fsplit-machine-function, the
user needs to add "-mllvm -mfs-psi-cutoff=0" to choose the right logic
for AutoFDO. The compiler should choose the right default values for
such case.

This CL separate MFS logic for different profile types.

Reviewed By: xur, wenlei

Differential Revision: https://reviews.llvm.org/D155253