[X86] Add a combine to turn (insert_subvector zero, (insert_subvector zero, X, Idx), Idx) into an insert of X into the larger zero vector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312460 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed