[SampleFDO] Flow Sensitive Sample FDO (FSAFDO) profile loader

This patch implements Flow Sensitive Sample FDO (FSAFDO) profile
loader. We have two profile loaders for FS profile,
one before RegAlloc and one before BlockPlacement.

To enable it, when -fprofile-sample-use=<profile> is specified,
add "-enable-fs-discriminator=true \
     -disable-ra-fsprofile-loader=false \
     -disable-layout-fsprofile-loader=false"
to turn on the FS profile loaders.

Differential Revision: https://reviews.llvm.org/D107878