Make the msse4/mno-sse4 flags aliases of the maximum sse values

they enable/disable.

This fixes two things:

a) sse4 isn't actually a target feature, don't treat it as one.
b) we weren't correctly disabling sse4.1 when we'd pass -mno-sse4
   after enabling it, thus passing preprocessor directives and
   (soon) passing the function attribute as well when we shouldn't.

llvm-svn: 233223