[LAA] Introduce enum for vectorization safety status (NFC).

This patch adds a VectorizationSafetyStatus enum, which will be extended
in a follow up patch to distinguish between 'safe with runtime checks'
and 'known unsafe' dependences.

Reviewers: anemet, anna, Ayal, hsaito

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D54892

llvm-svn: 349556