[PseudoProbe] Encode/Decode FS discriminator

Encoding FS discriminators for block probes. Decoding them correspondingly.

The encoding/decoding of FS discriminators are conditional, only for probes with a non-zero discriminator. This saves encoding size, also ensures downwards-compatiblity.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D147651