Formalize the notion that AVX and SSE are non-overlapping extensions from the compiler's point of view.  Per email discussion, we either want to always use VEX-prefixed instructions or never use them, and are taking "HasAVX" to mean "Always use VEX".  Passing -mattr=-avx,+sse42 should serve to restore legacy SSE support when desirable.

llvm-svn: 121439