Use compiler SSE2 definitions and C++11 alignas
Since C++11 alignas is a keyword, so we don't need to create our own.
All relevant compilers (gcc, clang, icc) define SSE2, so just use that.
There seem to be frequent crashes in the quad version of this code, but
this change should not make any difference from what I can tell.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: zzag, kwin
Differential Revision: https://phabricator.kde.org/D23120