[PATCH] Convert a vselect into a concat_vector if possible

[hfinkel at anl.gov]

Moving this to DAGCombine makes sense to me.

================
Comment at: lib/Target/X86/X86ISelLowering.cpp:17845
@@ +17844,3 @@
+    if (Cond->getOperand(i)->getOpcode() != ISD::UNDEF &&
+        Cond->getOperand(i).getNode() != BottomHalf)
+      return SDValue();
----------------
Does this do with right thing is BottomHalf is undef? It seems like you'd want to pick the first non-undef operand (and similarly with TopHalf below).

http://reviews.llvm.org/D3916