[PATCH][x86] Teach how to fold target specific dags for packed shifts by immedate count into build_vector.

[Nadav Rotem]

Overall the patch LGTM.  Do we have something like that for the non-target-specific code?  If not then we should move it there and figure out a way to handle the target-specific x86 nodes.  

+// Return true if this is a BUILD_VECTOR of ConstantSDNodes.
+// This is a helper function used by `getTargetVShiftByConstNode`.
+static bool isBuildVectorOfConstantSDNodes(SDValue N) {
+  if (N->getOpcode() != ISD::BUILD_VECTOR)
+    return false;
+
+  for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i)
+    if (!isa<ConstantSDNode>(N->getOperand(i)))
+      return false;
+  return true;
+}
+

You can also handle UNDEFs here.