[x86] avoid code explosion from LoopVectorizer for gather loop (PR27826)

By making pointer extraction from a vector more expensive in the cost model,
we avoid the vectorization of a loop that is very likely to be memory-bound:
https://llvm.org/bugs/show_bug.cgi?id=27826

There are still bugs related to this, so we may need a more general solution
to avoid vectorizing obviously memory-bound loops when we don't have HW gather
support.

Differential Revision: http://reviews.llvm.org/D20601

llvm-svn: 270729