Optimize vp9_highbd_block_error_8bit assembly.
A new version of vp9_highbd_error_8bit is now available which is optimized with AVX assembly. AVX itself does not buy us too much, but the non-destructive 3 operand format encoding of the 128bit SSEn integer instructions helps to eliminate move instructions. The Sandy Bridge micro-architecture cannot eliminate move instructions in the processor front end, so AVX will help on these machines. Further 2 optimizations are applied: 1. The common case of computing block error on 4x4 blocks is optimized as a special case. 2. All arithmetic is speculatively done on 32 bits only. At the end of the loop, the code detects if overflow might have happened and if so, the whole computation is re-executed using higher precision arithmetic. This case however is extremely rare in real use, so we can achieve a large net gain here. The optimizations rely on the fact that the coefficients are in the range [-(2^15-1), 2^15-1], and that the quantized coefficients always have the same sign as the input coefficients (in the worst case they are 0). These are the same assumptions that the old SSE2 assembly code for the non high bitdepth configuration relied on. The unit tests have been updated to take this constraint into consideration when generating test input data. Change-Id: I57d9888a74715e7145a5d9987d67891ef68f39b7