[openmp][amdgpu][nfc] Simplify implicit args handling

Removes a +x/-x pair on the only store/load of a variable
and deletes some nearby dead code. Also reduces the size of the implicit
struct to reflect the code currently emitted by clang.

Differential Revision: https://reviews.llvm.org/D114270

GitOrigin-RevId: 04954824ee158fffe6653c7eb51c07347b36ff21
3 files changed