i#1093 addendum: top 16 bits must be zero for x64
i#1093 addendum: top 16 bits must be zero for x64

SVN-Revision: 1975

bl4ckic3/dynamorio,sigma-random/dynamorio,code4bones/dynamorio,code4bones/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,bl4ckic3/dynamorio,sigma-random/dynamorio,daksunt/dynamorio,code4bones/dynamorio,code4bones/dynamorio,sigma-random/dynamorio,bl4ckic3/dynamorio