AMDGPU/GlobalISel: Start matching s_lshlN_add_u32 instructions

Use a hack to only enable this for GlobalISel.

Technically this also works with SelectionDAG, but the divergence
selection isn't reliable enough and a few cases fail, but I have no
desire to spend time writing the manual expansion code for it. The DAG
actually does a better job since it catches using v_add_lshl_u32 in
the mixed SGPR/VGPR cases.