i965/fs: Better heuristics on when to drop from SIMD16 to SIMD8, on register spilling
Submitted by Eero Tamminen
Assigned to Matt Turner
Currently SIMD16 versions of shaders are dropped on register spilling.
If same (number of) registers are spilled also by SIMD8 version, it most likely makes sense to keep the SIMD16 version.