[X86] Remove DAG combine expansion of extending masked load and truncating masked...

[X86] Remove DAG combine expansion of extending masked load and truncating masked store.

The only way to generate these was through promoting legalization
of narrow vectors, but we widen those types now. So we shouldn't
produce these nodes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368396 91177308-0d34-0410-b5e6-96231b3b80d8