[DAG] combineShuffleOfSplatVal - fold shuffle(splat,undef) -> splat, iff the splat...

[DAG] combineShuffleOfSplatVal - fold shuffle(splat,undef) -> splat, iff the splat contains no UNDEF elements

As noticed on D127115 - we were missing this fold, instead just having the shuffle(shuffle(x,undef,splatmask),undef) fold. We should be able to merge these into one using SelectionDAG::isSplatValue, but we'll need to match the shuffle's undef handling first.

This also exposed an issue in SelectionDAG::isSplatValue which was incorrectly propagating the undef mask across a bitcast (it was trying to just bail with a APInt::isSubsetOf if it found any undefs but that was actually the wrong way around so didn't fire for partial undef cases).