[asan] use ADD instead of OR when applying shadow offset of PowerPC. See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55975 for details

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173258 91177308-0d34-0410-b5e6-96231b3b80d8