legion: fix a race with index space nodes cleaning up derived expressions
legion: fix a race with index space nodes cleaning up derived expressions

StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion