AMDGPU/SI: Don't reserve XNACK when it's disabled

Summary:
This frees 2 additional scalar registers.

These are results from all of my 3 patches combined:

  Polaris:
    Spilled SGPRs: 2231 -> 1517 (-32.00 %)

  Tonga:
    Spilled SGPRs: 3829 -> 2608 (-31.89 %)
    Spilled VGPRs: 100 -> 84 (-16.00 %)

  Tonga even spills SGPRs via VGPRs to scratch. That's a compute shader
  limited to 64 VGPRs.

Reviewers: tstellarAMD

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tony-tye

Differential Revision: https://reviews.llvm.org/D27151

llvm-svn: 289262