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