[X86] Tweak avx512-gfni-intrinsics.ll tests to avoid xor(select(c,x,0)) 'passthrough' patterns

These can be manipulated by foldSelectWithIdentityConstant and lose the predicate/predicate-zero instruction test coverage - use an insertvalue chain into an aggregate instead to retain all the results.

Noticed while trying to convert foldSelectWithIdentityConstant to use llvm::isNeutralConstant