[SampleFDO] Enable FSAFDO loading passes if --enable-fs-discriminator is enabled

FSAFDO profile loader is currently disabled even --enable-fs-discriminator is enabled.
They need to be turned on by options which makes it cumbersome for experiments.

This patch changes the FSAFDO profile loader enabled by default.  Since they are
guarded by EnableFSDiscriminator, they will only be turned on if
--enable-fs-discriminator is enabled. Note that --enable-fs-discriminator is
still disabled by default.

Differential Revision: https://reviews.llvm.org/D119033