Check for less than 0 in shuffle mask instead of -1. It's more consistent with other...

Check for less than 0 in shuffle mask instead of -1. It's more consistent with other code related to shuffles and easier to implement in compiled code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172788 91177308-0d34-0410-b5e6-96231b3b80d8