[AbstractAttributor] Refine logic to indicate pessimistic fixed point when...

[AbstractAttributor] Refine logic to indicate pessimistic fixed point when folding `__kmpc_is_spmd_exec_mode`

Since we are using assumed information now, the logic should be refined to avoid
unncessary assertion.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D106630