Applies to speed >=1. Instead of searching all dual filter space
{R,Sm,Sh}x{R,Sm,Sh}, only check {R}x{R,Sm,Sh} followed by
{R,Sm,Sh}x{best of prev R,Sm,Sh}.

Saves ~6% of cycles by reducing av1_convolve_2d_sse2, with 0.023
overall psnr drop.

Change-Id: I82d7a6321b335293124a007ff4c87f0e260052e1