[X86] Add avx512vl and avx512dq command lines to combine-pmuldq.ll to...

[X86] Add avx512vl and avx512dq command lines to combine-pmuldq.ll to demonstrate where we fail to use pmuldq/pmuludq and use to pmullq instead.

It's nice that pmullq exists, but it has higher latency and probably lower throughput than pmuldq/pmuludq. We should prefer those if we can.

llvm-svn: 321436