[LV] For some IVs, use vector phis instead of widening in the loop body

Previously, whenever we needed a vector IV, we would create it on the fly,
by splatting the scalar IV and adding a step vector. Instead, we can create a
real vector IV. This tends to save a couple of instructions per iteration.

This only changes the behavior for the most basic case - integer primary
IVs with a constant step.

Differential Revision: http://reviews.llvm.org/D20315

llvm-svn: 271410