When refactoring vectorization flags, vectorization was disabled by default in the new pass manager.
This patch re-enables is for both managers, and changes the assumptions opt makes, based on the new defaults.
The patch also removes the -disable-loop-vectorization flag and leaves a single way to force *disable* vectorization for -O[23s] (via -vectorize-loops=false). We can add a cl::opt to force enabling under -O[01z] if anyone actually needs this.
Comments in opt.cpp should also help clarify the intended use of the flags to enable/disable vectorization.