Fix sse4 for target attribute feature additions.

Fix sse4 for target attribute feature additions.
This reinstates part of the hack removed in r233223, by special
casing sse4 as part of the feature additions. The notable change
here is that we consider it only as part of setting the SSE level
and not as part of the actual target features set which handles
setting the rest of the masks.

llvm-svn: 241130