TableGen/GlobalISel: Allow output instructions with multiple defs

TableGen/GlobalISel: Allow output instructions with multiple defs

The DAG behavior allows matchching input patterns with a single result
to the first result of an output instruction that defines multiple
results. The remaining defs are implicitly dead.

This starts to fix using manual selection for AMDGPU add/sub (although
it's still needed, mostly because it's also still needed for
G_PTR_ADD).