i965/fs: Better heuristics on when to drop from SIMD16 to SIMD8, on register spilling

Submitted by Eero Tamminen

Assigned to Matt Turner @mattst88

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.

Version: git

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