AMDGPU/SI: Emit the number of SGPR and VGPR spills

Summary:
v2: don't count SGPRs spilled to scratch twice

I think this is sufficient. It doesn't count private memory usage, which
happens often and uses scratch but isn't technically a spill. The private
memory usage can be computed by:
  [scratch_per_thread - vgpr_spills - a random multiple of SGPR spills].

The fact SGPR spills add very high numbers to the scratch size make that
computation a guessing game, but I don't have a solution to that.

Reviewers: tstellarAMD

Subscribers: arsenm, kzhuravl

Differential Revision: http://reviews.llvm.org/D22197

llvm-svn: 275288