nv50/ir: fix compilation warning in handleSharedATOM()

In release build mode only, op may be used uninitialized because
the assertion has been removed.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/codegen/nv50_ir_lowering_nvc0.cpp

@@ -1098,6 +1098,7 @@ NVC0LoweringPass::handleSharedATOM(Instruction *atom)
          break;
       default:
          assert(0);
+         return;
       }
 
       Instruction *i =