Revert "[DAGCombine] A shuffle of a splat is always the splat itself"

Revert "[DAGCombine]  A shuffle of a splat is always the splat itself"

This reverts commit r299047 which is incorrect because the
simplification may result in incorrect propogation of undefs to users of
the folded shuffle.

Thanks to Andrea Di Biagio for pointing this out.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299368 91177308-0d34-0410-b5e6-96231b3b80d8