AMDGPU/SI: Fix visit order assumption in SIFixSGPRCopies

Summary:
This pass was assuming that when a PHI instruction defined a register
used by another PHI instruction that the defining insstruction would
be legalized before the using instruction.

This assumption was causing the pass to not legalize some PHI nodes
within divergent flow-control.

This fixes a bug that was uncovered by r285762.

Reviewers: nhaehnle, arsenm

Subscribers: kzhuravl, wdng, nhaehnle, yaxunl, tony-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D26303

llvm-svn: 286676