[SampleFDO] Add two passes of MIRAddFSDiscriminatorsPass

This patch adds Pass1 of MIRADDFSDiscriminatorsPass before register
allocation, and Pass2 of MIRAddFSDiscriminatorsPass before
Block-Placement. This is still under --enable-fs-discrmininator
option (default false).

This would reduce the turn-around time for FSAFDO transition.

Differential Revision: https://reviews.llvm.org/D104579