[AMDGPU] Correct the handling of inlineasm output registers.

Summary:
- There's a regression due to the cross-block RC assignment. Use the
  proper way to derive the output register RC in inline asm.

Reviewers: rampitec, alex-t

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62537

llvm-svn: 361868