Silence gcc warning after r364458

Without the fix gcc 7.4.0 complains with

../lib/Target/X86/X86ISelLowering.cpp: In function 'bool getFauxShuffleMask(llvm::SDValue, llvm::SmallVectorImpl<int>&, llvm::SmallVectorImpl<llvm::SDValue>&, llvm::SelectionDAG&)':
../lib/Target/X86/X86ISelLowering.cpp:6690:36: error: enumeral and non-enumeral type in conditional expression [-Werror=extra]
             int Idx = (ZeroMask[j] ? SM_SentinelZero : (i + j + Ofs));
                        ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors

llvm-svn: 364507