[CGP] Allow cycles during Phi traversal in OptimizaMemoryInst

Allowing cycles in Phi traversal increases the scope of optimize memory instruction
in case we are in loop.

The added test shows an example of enabling optimization inside a loop.

Reviewers: loladiro, spatel, efriedma
Reviewed By: efriedma
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D35294

llvm-svn: 308419