[llvm] [PseudoProbe] Make probe discriminator compatible with dwarf base discriminator (PR #94506)

[Lei Wang via llvm-commits]

wlei-llvm wrote:

> 1. This change has zero impact on CSSPGO builds (both pass1 and pass2).
Tested on our internal services, both pass1 and pass2 are good.

> 2. Generally how many (%) of total dwarf discriminator can fit in the 3 bits.

Dumped the stats of this. So the the  3 bits.( <= 7)  discriminator account for roughly 96% of the total discriminator. 
Actually most are the zero discriminator about 70%,  value >=1 && value <=7 acount about 25%.  


https://github.com/llvm/llvm-project/pull/94506