[AMDGPU] move PHI nodes to AGPR class

If all uses of a PHI are in AGPR register class we should
avoid unneeded copies via VGPRs.

Differential Revision: https://reviews.llvm.org/D69200

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375297 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed