[AbstractAttributor] Fix two issues in folding __kmpc_is_spmd_exec_mode

This patch fixed two issues found when folding `__kmpc_is_spmd_exec_mode`:
1. When the reaching kernels are empty, it should not fold to generic mode.
2. When creating AA for the caller when updating information, the dependency
   should be required.

Reviewed By: ye-luo

Differential Revision: https://reviews.llvm.org/D106209