[MLIR][GPU] Add GPU ops nvvm.mma.sync, nvvm.mma.ldmatrix, lane_id

This change adds three new operations to the GPU dialect: gpu.mma.sync,
gpu.mma.ldmatrix, and gpu.lane_id. The former two are meant to target
the lower level nvvm.mma.sync and nvvm.ldmatrix instructions, respectively.
Lowerings are added for the new GPU operations for conversion to
NVVM.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D123647