Inline that uses temporary variables is currently initializing them
to 0 and passing in as output "+r".
This CL replaces the output constraint to "=&r" for most meaning an
output with early write (before inputs).  This allows the initialize
to zero step to be removed, saving 1 instruction.

BUG=libyuv:580
TESTED=local libyuv build on gcc/linux and try bots
R=harryjin@google.com

Review URL: https://codereview.chromium.org/1895743008 .