[X86][AVX512] Replace -cpu=knl with -mattr=+avx512f for avx512-intrinsics tests

It was noticed on D47377 that these tests were being unnecessarily affected by scheduler changes.

This adds vzeroupper at the end of some tests as we lose the 'FeatureFastPartialYMMorZMMWrite' feature from KNL, since Skylake+ don't support this its probably better.

llvm-svn: 333549