[AVX512] Handle valign masking intrinsic via C++ lowering

I think that this will scale better in most cases than adding a Pat<> for each
mapping from the intrinsic DAG to the intruction (i.e. rri, rrik, rrikz).  We
can just lower to the SDNode and have the resulting DAG be matches by the DAG
patterns.

Alternatively (long term), we could keep the Pat<>s but generate them via the
new AVX512_masking multiclass.  The difficulty is that in order to formulate
that we would have to concatenate DAGs.  Currently this is only supported if
the operators of the input DAGs are identical.