Silence gcc warning by adding parentheses to condition [NFC]

Without this gcc 7.4.0 complains with

  ../include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h:457:54: error: suggest parentheses around '&&' within '||' [-Werror=parentheses]
                    isArtifactCast(TmpDef->getOpcode()) &&
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
                        "Expecting copy or artifact cast here");
                        ~

llvm-svn: 365597