[DAGCombine] Avoid INSERT_SUBVECTOR reinsertions (PR28678)

[DAGCombine] Avoid INSERT_SUBVECTOR reinsertions (PR28678)

If the input vector to INSERT_SUBVECTOR is another INSERT_SUBVECTOR, and this inserted subvector replaces the last insertion, then insert into the common source vector.

i.e.
INSERT_SUBVECTOR( INSERT_SUBVECTOR( Vec, SubOld, Idx ), SubNew, Idx ) --> INSERT_SUBVECTOR( Vec, SubNew, Idx )

Differential Revision: https://reviews.llvm.org/D23330

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278211 91177308-0d34-0410-b5e6-96231b3b80d8