This will be used to check patterns referencing a forthcoming
INSERT_SUBVECTOR SDNode. INSERT_SUBVECTOR in turn is very useful for
matching to VINSERTF128 instructions and complements the already
existing EXTRACT_SUBVECTOR SDNode.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124145 91177308-0d34-0410-b5e6-96231b3b80d8