[AVX-512] Add support for turning a 256-bit load that goes to both halfs of an insert_subvector into a subvector broadcast.

Differential Revision: https://reviews.llvm.org/D25650

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@284353 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed