Add 4-tap version of 2nd-pass ARMv6 MC filter.
The existing code applied a 6-tap filter with 0's on either end. We're already paying the branch penalty to avoid computing the two extra columns needed as input to this filter. We might as well save time computing the filter as well. This reduces the inner loop from 21 instructions to 16, the number of loads per iteration from 4 to 1, and the number of multiplies from 7 to 4. The gain in overall decoding performance, however, is small (less than 1%). This change also means we now valgrind clean on ARMv6, which is its real purpose. The errors reported here were valgrind's fault (it does not detect that 0 times an uninitialized value is initialized), but Julian Seward says it would slow down valgrind considerably to make such checks. Speeding up libvpx rather, even by a small amount, seems a much better idea if only to enable proper valgrind checking of the rest of the codec. Change-Id: Ifb376ea195e086b60f61daf1097d8910c4d8ff16