[CSSPGO] Exclude pseudo probes from slot index

Pseudo probe are currently given a slot index like other regular instructions. This affects register pressure and lifetime weight computation because of enlarged lifetime length with pseudo probe instructions. As a consequence, program could get different code generated w/ and w/o pseudo probes. I'm closing the gap by excluding pseudo probes from stack index and downstream register allocation related passes.

Reviewed By: wmi

Differential Revision: https://reviews.llvm.org/D100334