[LV][VPlan] Detect outer loops for explicit vectorization.

Patch #2 from VPlan Outer Loop Vectorization Patch Series #1
(RFC: http://lists.llvm.org/pipermail/llvm-dev/2017-December/119523.html).

This patch introduces the basic infrastructure to detect, legality check
and process outer loops annotated with hints for explicit vectorization.
All these changes are protected under the feature flag
-enable-vplan-native-path. This should make this patch NFC for the existing
inner loop vectorizer.

Reviewers: hfinkel, mkuper, rengolin, fhahn, aemerson, mssimpso.

Differential Revision: https://reviews.llvm.org/D42447

llvm-svn: 330739