[PowerPC] Check if the pre-increment PHI Node already exists

Preparations to use the per-increment are sometimes done in the target
independent pass Loop Strength Reduction. We try to detect them in the PowerPC
specific pass so that they are not done twice and so that we do not add PHIs
that are not required.

Differential Revision: https://reviews.llvm.org/D36736

llvm-svn: 311332