R600: Modernize work item intrinsics test

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewed-by: Matt Arsenault <Matthew.Arsenault@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214451 91177308-0d34-0410-b5e6-96231b3b80d8
Make classof in MemSDNode consistent with MemIntrinsicSDNode

If INTRINSIC_W_CHAIN and INTRINSIC_VOID are MemIntrinsicSDNodes, and a
MemIntrinsicSDNode is a MemSDNode, then INTRINSIC_W_CHAIN and INTRINSIC_VOID
must be MemSDNodes too.

Noticed by inspection.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214452 91177308-0d34-0410-b5e6-96231b3b80d8