[X86] CombineOr - fold to generic funnel shifts

As discussed on D57389, this is a first step towards moving the SHLD/SHRD matching code to DAGCombiner using FSHL/FSHR instead.

There's a bit of work to do before I can do that, so this just folds to FSHL/FSHR in the existing code (handling the different SHRD/FSHR argument ordering), which fixes the issue we had with i16 shift amounts not being correctly masked.

llvm-svn: 353626