mips: add a missing break

Add a missing break found by IBM's BEAM checker and reported by Florian.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2972 8f6e269a-dfd6-0310-a8e1-e2731360e62c
diff --git a/priv/host_mips_isel.c b/priv/host_mips_isel.c
index fd32ff4..c3bbd9a 100644
--- a/priv/host_mips_isel.c
+++ b/priv/host_mips_isel.c
@@ -3954,6 +3954,7 @@
             addInstr(env, MIPSInstr_Cas(4, old, addr, expd, data, mode64));
          }
       }
+      return;
 
    /* --------- INSTR MARK --------- */
    /* Doesn't generate any executable code ... */