[X86] Pull out constant splat rotation detection.

We had 3 different approaches - consistently use getTargetConstantBitsFromNode and allow undef elts.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349319 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed