[AVX512] Change predicates on some vXi16/vXi8 AVX store patterns so they stay...

[AVX512] Change predicates on some vXi16/vXi8 AVX store patterns so they stay enabled unless VLX and BWI instructions are supported."

Without this we could fail instruction selection if VLX was enabled, but BWI wasn't.

llvm-svn: 268885