AMDGPU/SI: Avoid creating unnecessary copies in the SIFixSGPRCopies pass

Summary:
1. Don't try to copy values to and from the same register class.
2. Replace copies with of registers with immediate values with v_mov/s_mov
   instructions.

The main purpose of this change is to make MachineSink do a better job of
determining when it is beneficial to split a critical edge, since the pass
assumes that copies will become move instructions.

This prevents a regression in uniform-cfg.ll if we enable critical edge
splitting for AMDGPU.

Reviewers: arsenm

Subscribers: arsenm, kzhuravl, llvm-commits

Differential Revision: https://reviews.llvm.org/D23408

llvm-svn: 287131