Extend the SSE2 comment lexing to AVX2. Only 16byte align when not on AVX2.

This provides some 3% speedup when preprocessing gcc.c as a single file.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205436 91177308-0d34-0410-b5e6-96231b3b80d8