[LV] Add -scalable-vectorization=<option> flag.

This patch adds a new option to the LoopVectorizer to control how
scalable vectors can be used.

Initially, this suggests three levels to control scalable
vectorization, although other more aggressive options can be added in
the future.

The possible options are:
- Disabled:   Disables vectorization with scalable vectors.
- Enabled:    Vectorize loops using scalable vectors or fixed-width
              vectors, but favors fixed-width vectors when the cost
              is a tie.
- Preferred:  Like 'Enabled', but favoring scalable vectors when the
              cost-model is inconclusive.

Reviewed By: paulwalker-arm, vkmr

Differential Revision: https://reviews.llvm.org/D101945