deltaIRStmt: handle IRStmt_MFence.


git-svn-id: svn://svn.valgrind.org/vex/trunk@1238 8f6e269a-dfd6-0310-a8e1-e2731360e62c
diff --git a/priv/ir/iropt.c b/priv/ir/iropt.c
index d8b7c38..af595ef 100644
--- a/priv/ir/iropt.c
+++ b/priv/ir/iropt.c
@@ -2827,6 +2827,7 @@
    switch (st->tag) {
       case Ist_NoOp:
       case Ist_IMark:
+      case Ist_MFence:
          break;
       case Ist_AbiHint:
          deltaIRExpr(st->Ist.AbiHint.base, delta);