[FS-AFDO] Clean up non-zero discriminator for pseudo probes at the first FS discriminator pass.

The dwarf discriminator field for pseudo probes is not supposed to be used until the first FS discriminator pass. Unfortunately there are always corner cases that accidientally set this field. For example, the inliner could set this field for an inlined instruction if the instruction does not come with any debug information. While fixing all such spots is possible, but for future-proff I'd like to enforce a general cleanup before assigning probes any FS discriminator.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D150741