Enable LoopVectorization by default.

Summary:
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.
Comments in opt.cpp should clarify the intended use of all flags to enable/disable vectorization.

Reviewers: chandlerc, jgorbe

Subscribers: jlebar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61091

llvm-svn: 359167