PixelShaderGen: Cleanups and fixes for tev combiners.

Fixes issue 4674.
PixelShaderGen: Implement tev combiner lerping in a faster way which also reproduces hardware behavior perfectly.

The new behavior has been verified to be correct by hardware tests. This is an improvement over the old code, which was just a guess.
PixelShaderGen: Cleanups.
Software renderer: Use color combiner configuration for alpha combiner compare mode inputs.

As pointed out by dolphin-emu/hwtests@f684f2498.
Software renderer: Properly calculate tev combiner output.

As pointed out by dolphin-emu/hwtests@461476112.
Merge pull request #193 from neobrain/tev_combiner_fixes

PixelShaderGen: Cleanups and fixes for tev combiners.