mesa: add switch case for ir_txf_ms to silence warning

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/program/ir_to_mesa.cpp

@@ -2045,6 +2045,9 @@ ir_to_mesa_visitor::visit(ir_texture *ir)
       ir->lod_info.grad.dPdy->accept(this);
       dy = this->result;
       break;
+   case ir_txf_ms:
+      assert(!"Unexpected ir_txf_ms opcode");
+      break;
    }
 
    const glsl_type *sampler_type = ir->sampler->type;