[SelectionDAG] Add common vector constant folding helper function

We have a number of functions that implement constant folding of vectors (unary and binary ops) in near identical manners (and the differences don't appear to be critical).

This patch introduces a common implementation (SelectionDAG::FoldConstantVectorArithmetic) and calls this in both the unary and binary op cases.

After this initial patch I intend to begin enabling vector constant folding for a wider number of opcodes in SelectionDAG::getNode().

Differential Revision: http://reviews.llvm.org/D13665

llvm-svn: 250118