[AMDGPU] Add __builtin_amdgcn_grid_size

[AMDGPU] Add __builtin_amdgcn_grid_size

Similar to D76772, loads the data from the dispatch pointer. Marked invariant.

Patch also updates the openmp devicertl to use this builtin.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D90251