[OpenMP][Fix] Fix disable spmdization option

Besides SPMDization, other analysis and optimization for original, frontend-generated SPMD regions uses information from the AAKernelInfoFunction attribute. This fix makes sure disabling SPMDization through the corresponding option applies only to generic mode regions, which should not be SPMDized, while it leaves unaffected the attribute state of original SPMD regions.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D108001