R600/SI: allow 5 more input SGPRs to a shader

Our OpenGL driver needs 22 SGPRs (16 user SGPRs + 6 streamout non-user SGPRs).

Signed-off-by: Marek Olšák <marek.olsak@amd.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207990 91177308-0d34-0410-b5e6-96231b3b80d8