Rewrite vp8_short_walsh4x4_sse2()

This rewriting reflects changes made in commit "Improve the
accuracy of forward walsh-hadamard transform". Since this function
is not called much, only a small encoder performance gain (~0.5% )
is seen.

Change-Id: Ie9df58a43028a11fd5b115c4bbe3141f7596578b
2 files changed