[x86] Revert r212324 which was too aggressive w.r.t. allowing undef
lanes in vector splats.

The core problem here is that undef lanes can't *unilaterally* be
considered to contribute to splats. Their handling needs to be more
cautious. There is also a reported failure of the nightly testers
(thanks Tobias!) that may well stem from the same core issue. I'm going
to fix this theoretical issue, factor the APIs a bit better, and then
verify that I don't see anything bad with Tobias's reduction from the
test suite before recommitting.

Original commit message for r212324:
  [x86] Generalize BuildVectorSDNode::getConstantSplatValue to work for
  any constant, constant FP, or undef splat and to tolerate any undef
  lanes in a splat, then replace all uses of isSplatVector in X86's
  lowering with it.

  This fixes issues where undef lanes in an otherwise splat vector would
  prevent the splat logic from firing. It is a touch more awkward to use
  this interface, but it is much more accurate. Suggestions for better
  interface structuring welcome.

  With this fix, the code generated with the widening legalization
  strategy for widen_cast-4.ll is *dramatically* improved as the special
  lowering strategies for a v16i8 SRA kick in even though the high lanes
  are undef.

  We also get a slightly different choice for broadcasting an aligned
  memory location, and use vpshufd instead of vbroadcastss. This looks
  like a minor win for pipelining and domain crossing, but a minor loss
  for the number of micro-ops. I suspect its a wash, but folks can
  easily tweak the lowering if they want.