[AMDGPU] Fix SGPR fixing through SCC chaining

[AMDGPU] Fix SGPR fixing through SCC chaining

Summary:
- During the fixing of SGPR copying from VGPR, ensure users of SCC is
  properly propagated, i.e.
  * only propagate through live def of SCC,
  * skip the SCC-def inst itself, and
  * stop the propagation on the other SCC-def inst after checking its
    SCC-use first.

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59362

llvm-svn: 356258