[Sample-PGO] Emit FS discriminators only when -fdebug-info-for-profiling is set

IR level addDiscriminator pass is guarded by DebugInfoForProfiling
(set by option -fdebug-info-for-profiling).
This patch syncs the logic for the MIR and IR level implementations.

Differential Revision: https://reviews.llvm.org/D120536