AMDGPU/SI: Don't fix a PHI under uniform branch in SIFixSGPRCopies only when...

AMDGPU/SI: Don't fix a PHI under uniform branch in SIFixSGPRCopies only when sources and destination are all sgprs

Summary:
  If a PHI has at lease one VGPR operand, we have to fix the PHI
in SIFixSGPRCopies.

Reviewer:
  Matt

Differential Revision:
  http://reviews.llvm.org/D34727

llvm-svn: 309959