[LoopUtils, LV] Fix PR27246 (first-order recurrences)

This patch ensures that when we detect first-order recurrences, we reject a phi
node if its previous value is also a phi node. During vectorization the initial
and previous values of the recurrence are shuffled together to create the value
for the current iteration. However, phi nodes are not widened like other
instructions. This fixes PR27246.

Differential Revision: http://reviews.llvm.org/D18971

llvm-svn: 265983