Formalize the notion that AVX and SSE are non-overlapping extensions from the compile...

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121439 91177308-0d34-0410-b5e6-96231b3b80d8