Use a shuffle with undef elements instead of inserting 0s in the 128-bit to 256-bit casting intrinsics to improve performance. Thanks to Katya Romanova for identifying this issue.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187716 91177308-0d34-0410-b5e6-96231b3b80d8