[GlobalISel] fix a compilation error with gcc 6.3.0

With gcc 6.3.0, I hit the following compilation error:
  ../lib/CodeGen/GlobalISel/Combiner.cpp: In member function
      ‘bool llvm::Combiner::combineMachineInstrs(llvm::MachineFunction&,
       llvm::GISelCSEInfo*)’:
  ../lib/CodeGen/GlobalISel/Combiner.cpp:156:54: error: suggest parentheses
       around ‘&&’ within ‘||’ [-Werror=parentheses]
     assert(!CSEInfo || !errorToBool(CSEInfo->verify()) &&
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
                            "CSEInfo is not consistent. Likely missing calls to "
                            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                            "observer on mutations");

Fix the code as suggested by the compiler.