[AMDGPU] Fix crash in phi-elimination hook.

Summary: - Pre-check in case there's just a single PHI insn.

Reviewers: alex-t, rampitec, arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, dstuttard, tpr, t-tye, hiraditya, llvm-commits, yaxunl

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67451

llvm-svn: 371649