opt: Mirror vectorization presets of clang

clang enables vectorization at optimization levels > 1 and size level < 2. opt
should behave similarily.

Loop vectorization and SLP vectorization can be disabled with the flags
-disable-(loop/slp)-vectorization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196294 91177308-0d34-0410-b5e6-96231b3b80d8