[LV] Enable vectorization of loops where the IV has an external use

Vectorizing loops with "escaping" IVs has been disabled since r190790, due to
PR17179. This re-enables it, with support for external use of both
"post-increment" (last iteration) and "pre-increment" (second-to-last iteration)
IVs.

Differential Revision: http://reviews.llvm.org/D21048

llvm-svn: 272715