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