[PseudoProbe] Only emit discriminstor in FS-AFDO mode.

Despite previous effort {D148569} to avoid screwing up existing disrminator field, I'm still seeing some call probes getting a non-zero discriminator eventually in non-FS mode. It could be related to callsite merge. While they are investigated I'm disabling discriminator emission for non-FS mode. This avoids breaking the compatiblity with older tools like llvm-profgen and bolt.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D150625