[X86] AVX512: Only allow k1-k7 as predicates to vpcmp*

As destination k0 is allowed but not as predicate/writemask.

I also modified the test to allow checking of error messages by the assembler.
I applied a similar approach to the test ret.s in the same directory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212504 91177308-0d34-0410-b5e6-96231b3b80d8