Skip DBG instr in OptimizePHIs when looking for dead PHI cycles

Skip DBG instr in OptimizePHIs when looking for dead PHI cycles

Summary:
Changed use_instructions() to use_nodbg_instructions() when
building an instruction set.

We don't want the presence of debug info to affect the code
we generate.

Reviewers: dblaikie, Eugene.Zelenko, chandlerc, aprantl

Reviewed By: aprantl

Subscribers: aprantl, llvm-commits

Differential Revision: https://reviews.llvm.org/D40882

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@320010 91177308-0d34-0410-b5e6-96231b3b80d8