Don't clobber q4-q7 in WelsIntra16x16Combined3Satd_neon

This is similar to what is done in other neon functions. This
function was missed since it isn't covered by the current
set of unittests.
Merge pull request #521 from mstorsjo/neon-register-backup

Don't clobber q4-q7 in WelsIntra16x16Combined3Satd_neon