[PATCH] Lower certain build_vectors to insertps instructions

[Elena Demikhovsky]

Let's assume that you need to insert 2 or 3 elements that extracted from vector X.
But you create an INSERPS node that puts one element to UNDEF.

return DAG.getNode(X86ISD::INSERTPS, dl, VT, V, DAG.getUNDEF(VT),
                     InsertpsMask);

-  Elena

http://reviews.llvm.org/D3521