[NVPTX] Remove MemIntrinsicSDNode/MemSDNode duplicate checking

As of r214452, isa<MemSDNode> will return true for nodes for which
isa<MemIntrinsicSDNode> will return true (classof now respects the actual class
hierarchy). So we no longer need to check for both MemIntrinsicSDNode and
MemSDNode separately.

No functionality change intended.