vp9lpf/x86: store unpacked intermediates for filter6/14 on stack.

filter16 goes from 508 to 482 (h) or 346 to 314 (v) cycles; filter88
goes from 240 to 238 (h) or 174 to 165 (v) cycles, measured on TOS.

Signed-off-by: Anton Khirnov <anton@khirnov.net>