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Patch https://aomedia-review.googlesource.com/c/10901/ temporarily
disabled the SSE2 warp filter for 4x4 blocks, because of a
data race when the filter was used at the right-hand edge of a
tile in a multithreaded encode.

This patch fixes the data race and re-enables the SSE2 warp filter.

Change-Id: I7058c897ddf538cd10001c5be13b1a1bfe8320fd