[x86] fix cost model inaccuracy for vector memory ops

The irony of this patch is that one CPU that is affected is AMD Jaguar, and Jaguar
has a completely double-pumped AVX implementation. But getting the cost model to
reflect that is a much bigger problem. The small goal here is simply to improve on
the lie that !AVX2 == SandyBridge.

Differential Revision: http://reviews.llvm.org/D18000

llvm-svn: 263069