That can usually be lowered efficiently and is common in sandybridge code.
It would be nice to do this in DAGCombiner but we can't insert arbitrary
BUILD_VECTORs this late.
Fixes PR15462.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176634 91177308-0d34-0410-b5e6-96231b3b80d8